Diving Glossary

A reference for the terms used across this site, from your first BCD purchase to the politics of the next Hope Spot.

Diving Basics

Open Water Diver: The entry-level scuba certification. Allows divers to dive to 18 metres without supervision. Issued by PADI, SSI, NAUI, BSAC, and other agencies. Recognised globally for life.
Advanced Open Water: The next certification after Open Water. Extends the depth limit to 30 metres and adds five adventure dives, including mandatory Deep and Underwater Navigation. Required for many serious dive sites.
Rescue Diver: A certification focused on managing diver emergencies, self-rescue, and assisting other divers. The course required before progressing to Divemaster or Instructor levels.
Divemaster: The first professional-level scuba certification. Divemasters can guide certified divers and assist instructors. Caro is a PADI Divemaster.
DSD: Discover Scuba Diving. A short, instructor-supervised first-time experience that lets non-divers try diving in a pool and shallow open water. Not a certification.
Open Water depth limit: 18 metres. The maximum depth a non-Advanced certified diver should descend to.
Recreational depth limit: 40 metres. The depth beyond which divers cross into technical diving territory and need specialist training and gas mixes.

BCD: Buoyancy Compensator Device. The inflatable jacket that controls a diver's buoyancy and holds the tank in position. Three main styles: jacket, wing (back-inflate), and travel.
Regulator: The mechanism that converts the high-pressure air in the tank into breathable pressure for the diver. Includes a first stage attached to the tank and a second stage in the diver's mouth.
Dive Computer: An electronic device worn on the wrist or in a console that tracks depth, time, and nitrogen loading on each dive. Replaces dive tables for managing decompression risk.
SMB: Surface Marker Buoy. An inflatable signalling buoy a diver deploys before surfacing to alert boats and crew to the ascent point. Especially important in current-exposed open water.
PLB: Personal Locator Beacon. A GPS-enabled device a diver carries that activates if separated from the boat, broadcasting position to rescue services. Mandatory at Malpelo following the 2016 Maria Patricia incident.
Nitrox: Enriched Air Nitrox. A breathing gas with elevated oxygen content (typically 32 to 40 percent), allowing longer no-decompression bottom times. Requires a separate certification.
Wetsuit: A neoprene exposure suit that traps a thin layer of water against the body and uses body heat to warm it. Thickness varies from 3mm in tropical water to 7mm in cold.

Negative Entry: A descent technique where a diver enters the water and immediately drops below the surface without inflating the BCD. Used in current-exposed sites where the surface current would otherwise sweep divers off the dive site.
Equalisation: Releasing pressure built up in the ear canals during descent by gently exhaling against pinched nostrils. Failure to equalise causes barotrauma. Can be learned but cannot be skipped.
Buddy Check: Pre-dive safety verification of a partner's gear: BCD, weights, releases, air source, final OK. The single most effective dive safety practice.
Safety Stop: A standard three-minute pause at five metres at the end of any dive deeper than 10 metres, allowing nitrogen to off-gas before surfacing.
Bottom Time: The total time spent below the surface on a single dive, measured from descent to ascent. Limited by depth and breathing gas.

Hammerhead Shark: A family of sharks distinguished by their flattened, T-shaped heads. Scalloped hammerheads aggregate at sites like Malpelo, Cocos, and the Galápagos in schools that can number in the hundreds.
Whale Shark: The world's largest fish, growing to 12 metres or more. Filter feeders, harmless to humans. Reliably sighted at Koh Tao's pinnacle sites and at Malpelo year-round.
Bull Shark: A thick-bodied requiem shark with a reputation built partly on its ability to enter brackish water. Aggregates in winter off Playa del Carmen, Mexico, where pregnant females gather between November and March.
Smalltooth Sand Tiger Shark: Odontaspis ferox. A deep-water shark and Malpelo's signature cold-water specialty. Most often seen during the January-to-April upwelling season when they rise to depths divers can reach.
Tiger Shark: Galeocerdo cuvier. A large striped requiem shark, historically rare at Malpelo but increasingly sighted in recent years.
Manta Ray: Large filter-feeding rays with wingspans up to seven metres. Aggregate at cleaning stations in tropical waters. Common encounter at Elphinstone Reef in Egypt.
Dugong: A marine mammal related to manatees, found in the Indo-Pacific. Abu Dabbab in Marsa Alam is one of the best places in the Red Sea to encounter them in seagrass beds.

Marine Protected Area (MPA): A defined area of ocean where extractive activities are restricted or banned to allow ecosystems to recover. The most effective MPAs are no-take zones with active enforcement.
Hope Spot: A designation by Mission Blue, the organisation founded by Dr Sylvia Earle, identifying critical ocean areas that warrant protection. Egypt's Great Fringing Reef and Rapa Nui are recent examples.
30 by 30: The global commitment to protect 30 percent of land and ocean by 2030, agreed under the Kunming-Montreal Global Biodiversity Framework at COP15 in 2022.
Coral Bleaching: When corals expel the symbiotic algae living in their tissues due to thermal or chemical stress, turning them white. Bleached corals can recover if conditions improve, but prolonged bleaching kills the colony.
Fourth Global Bleaching Event: The bleaching event running through 2024 and 2025, the largest on record, affecting reef systems on every continent with major reef habitat. The fourth such global event since records began in 1998.
High Seas Treaty: The Biodiversity Beyond National Jurisdiction (BBNJ) Agreement, formally entered into force in January 2026. Creates the legal mechanism for designating marine protected areas in international waters.

Apnea: The Spanish and technical name for freediving. Diving on a single breath without breathing equipment.
AIDA: International Association for the Development of Apnea. The most globally recognised freediving certification body and the agency behind major international competitions.
Molchanovs: A freediving certification system built around the methodology of the Molchanov family. The fastest-growing alternative to AIDA among recreational and competitive freedivers.
Static Apnea: A freediving discipline where the diver holds breath while floating face-down, motionless. Used in training and competition to develop breath capacity and mental control.
Diaphragm Contractions: Involuntary contractions of the diaphragm during a long breathhold, signalling the body's CO2 build-up. Trained freedivers learn to stay relaxed through them rather than ending the dive.