densityMass per unit of volume: why cold, salty water sinks. Seawater at the surface averages 1.025 kilograms per liter, against 1.0 for fresh water.
gyreA giant loop of currents circling an ocean basin. There are five major gyres: two in the Pacific, two in the Atlantic and one in the Indian Ocean.
marine biologyThe study of sea plants and sea animals. Much of Earth's life is in the ocean, and many of its species have not yet been discovered.
ocean currentA stream within the sea that keeps to a set course. Wind, tides and differences in the water's temperature and saltiness drive currents. Speeds are given in knots or in meters per second.
oceanographyThe work of scientists who study the world's seas. Other names for it are oceanology and marine science. The Challenger expedition of 1872 to 1876, the first true oceanographic cruise, is counted as the start of the modern science.
salinitySaltiness, given in parts per thousand. Seawater averages about 35 parts per thousand, or 3.5 percent salt by weight. It is generally lower at the equator and the poles than at mid-latitudes.
Show all 36 terms (30 more)
Ocean basics, continued
seawaterThe salty water that fills the oceans. Chloride and sodium make up about 85 percent of its dissolved ions.
sonarMapping and detecting by timing echoes in the sea. The letters stand for sound navigation and ranging. Active sonar sends out a pulse and times the echo; passive sonar only listens.
thermoclineThe zone below the surface where warmth drops away. Its depth changes with the season and the year. In the tropics it is nearly permanent, and near the poles it is shallow or missing.
tideThe slow, regular swing of the water line at a coast. Tides answer to the gravity of the moon and of the sun. They are in fact very long waves: high tide is the crest and low tide the trough.
upwellingDeep water moving upward to fill a gap at the top. The water that comes up is rich in nutrients, so good fishing grounds are often found where upwelling is common.
waveA ridge of water that the wind raises and sends along. A wave carries energy, not water, across the sea. Waves can travel thousands of kilometers before they reach land.
Waves and tides 12
crestThe peak of a single wave, before the water dips. When two crests of the same size meet, the water rises twice as high.
fetchThe length of uninterrupted water under one wind. The longer the fetch and the stronger the wind, the larger the waves.
longshore currentA current that runs along a beach, parallel to shore. It forms because waves usually reach the beach at a slight angle. The sediment it moves along the coast is called longshore drift.
neap tideThe tide at its weakest, a week after its strongest. It comes at a half moon, the first or the third quarter, when the moon pulls at right angles to the sun. High tides are then a little lower and low tides a little higher than average.
rip currentA fast channel of water heading out through the surf. It can move at up to eight feet per second, faster than an Olympic swimmer. A rip tide is a different thing, a tidal flow through an inlet.
spring tideThe tide at its strongest, when sun and moon line up. The name is not about the season: the tide is said to spring forth. It comes twice each lunar month, at about the time of a new or a full moon.
storm surgeWater piled against a coast by the winds of a storm. Its size is the gap between the water's actual height and the tide that was predicted. Storm tide is the surge and the normal tide together.
swellEvenly spaced waves that outran a faraway storm. Swell comes from distant weather systems. Its waves typically have long wavelengths.
tidal rangeHow far the sea drops from high tide to low tide. The Bay of Fundy in Canada has the largest in the world, 11.7 meters on average.
troughThe low point between one wave and the next. When a trough meets a crest of the same size, the two cancel out and the water lies flat.
wave heightHow far a crest stands above the trough beside it. In storms it may exceed 10 meters, or 33 feet. Forecasts give the significant wave height, the average of the highest third of the waves.
wave periodThe time between one crest passing and the next. It is timed at a fixed point as the waves go by.
Depth and tools 12
brackish waterWater with some salt, but less than the open sea. It forms where seawater and fresh water mix, as in an estuary. The word goes back to brak, in Middle Dutch.
Coriolis effectThe bending of moving water and air by Earth's spin. North of the equator it turns winds and currents to the right, and south of it to the left.
dissolved oxygenOxygen held in water, which water animals breathe. Water takes it up from the air and from water plants. A place where it runs very low is called a dead zone.
downwellingSurface water sinking toward the deep. Along a coast it happens when wind piles surface water up against the shore. It is the reverse of upwelling.
haloclineA layer where fresher water rests on saltier water. The word comes from Greek words for salt and slope. Haloclines are common where fresh water from rivers or melting ice meets salty ocean water.
hydrophoneA microphone made to pick up sound underwater. Most use a ceramic that gives a small electric signal when the pressure on it changes. Arrays of them record underwater volcanoes, earthquakes and icequakes.
mixed layerThe sea's uppermost water, kept uniform by stirring. Wind and waves do the stirring and spread the warmth. The thermocline lies at its base.
pycnoclineA layer of fast-changing density that resists mixing. It lies below the mixed layer and separates the upper water from the lower.
research vesselA ship fitted out for scientific work at sea. Some are built on icebreaker hulls so that they can work in polar waters.
thermohaline circulationDeep ocean flow driven by temperature and salt. Thermo stands for temperature and haline for salt. Its currents move much more slowly than tidal or surface currents.
tide gaugeAn instrument that keeps a record of the water level. Older gauges used a float in a well. Modern stations time a sound signal reflected from the water's surface and take a reading every six minutes.
water massA body of seawater with its own temperature and salt. Oceanographers tell water masses apart by temperature, salinity and other tracers. North Atlantic Deep Water is one of them.
How this list was compiled: Term list chosen for this site on 2026-10-15. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Oceanography (its other names and the Challenger expedition), Ocean current, Tide, Salinity, Upwelling, Wind wave (wave height, wave period and significant wave height), Ocean gyre, Sonar, Thermocline, Seawater, Marine biology, Density, Crest and trough, Wave height, Swell (ocean), Wind fetch, Tidal range (the Bay of Fundy), Rip current, Longshore drift, Storm surge, Thermohaline circulation, Coriolis force, Downwelling, Halocline, Pycnocline, Water mass, Mixed layer, Research vessel, Tide gauge, Hydrophone and Brackish water. Spring tide, neap tide, crest, trough, seawater and downwelling were also read in Wiktionary, the free dictionary (https://en.wiktionary.org/). Checked a second time against the Ocean Facts pages of NOAA's National Ocean Service (https://oceanservice.noaa.gov/facts/): What is a current?, What are tides?, What is a gyre?, What is sonar?, What is a rip current?, What are spring and neap tides?, Why does the ocean have waves?, Why is the ocean salty? (35 parts per thousand, chloride and sodium), What is storm surge?, What is upwelling? (and downwelling), What is a thermocline? (and the mixed layer), What is the global ocean conveyor belt?, Why does the ocean get colder at depth?, What does an oceanographer do? (the four branches), What is a tide gauge?, What is a hydrophone? and What is a dead zone?; and against its Currents tutorial (https://oceanservice.noaa.gov/education/tutorial_currents/): Longshore Currents, Surface Ocean Currents (the Coriolis effect on winds and currents) and The Ekman Spiral. Wave height and its 10 meters in storms are from How are ocean waves described? of the NOAA National Data Buoy Center (https://www.ndbc.noaa.gov/educate/waves.shtml), and dissolved oxygen from Indicators: Dissolved Oxygen of the U.S. Environmental Protection Agency (https://www.epa.gov/national-aquatic-resource-surveys/indicators-dissolved-oxygen). The fact after each meaning is from the same pages. Licence: own work
How it works
This oceanography vocabulary worksheet prints oceanography, ocean current, tide, salinity, upwelling, wave, gyre, sonar, thermocline and three more terms in one column, with their meanings in another order beside them. A key follows on the next page.
Read the full guide
Waves and tides
Set opens on Ocean basics. The second set has crest, trough, wave height, wave period, fetch, swell, spring tide, neap tide, tidal range, rip current, longshore current and storm surge.
Depth and tools
The third set has thermohaline circulation, Coriolis effect, downwelling, halocline, pycnocline, water mass, mixed layer, dissolved oxygen, research vessel, tide gauge, hydrophone and brackish water.
Trench, seamount and continental shelf are terms on the ocean floor vocabulary worksheet, not here.
Questions and answers
What are the 4 branches of oceanography?
Biological oceanography studies the plants and animals of the sea. Chemical oceanography studies what seawater is made of. Geological oceanography studies the ocean floor. Physical oceanography studies waves, currents and tides and the way ocean and atmosphere act on each other.
Why is the ocean salty?
Rain is slightly acidic and wears away rock on land. Rivers carry the dissolved salts to the sea, where many of them stay and build up over time. Vents in the seafloor add more.
Why does the ocean get colder with depth?
Cold, salty water is dense, so it sinks to the bottom of the ocean basins and lies under the warmer, less dense water near the surface. Heat from inside the Earth reaches the seafloor too, but far too little to warm the deep sea.