The term glider may refer to both wave glider (discussed above in the Autonomy section) and ocean glider. Unlike a wave glider, an ocean glider IS often considered an AUV as it is both autonomous and operates completely underwater. Ocean gliders use changes in buoyancy to move vertically, and (like wave gliders) use ocean currents to move horizontally. They are therefore similar to air gliders, except that ocean gliders have as little difficulty moving up as they do down. Like wave gliders, ocean gliders can travel for long distances as they do not require much power (only to make small adjustments in their buoyancy). A good explanation of an ocean glider and one of the most widely used types (the Slocum glider) can be found at a website [6] produced by the National Oceanic and Atmospheric Administration (NOAA). 

​Gliders are often considered as a separate oceanographic vehicle category, as they usually move passively; that is, they rely on buoyancy and water movement to travel, whereas other AUVs use active propulsion (i.e. using a diesel engine, batteries, solar energy). The idea of separating vehicles based on their ability to move independently is comparable to how we separate biology in the ocean: phytoplankton (from the Greek words for "plant" and "drifter") move in the same way as gliders, whereas their predators, zooplankton (from the Greek words for "animal" and "drifter") are capable of swimming and can thus propel themselves independently, similar to AUVs and ROVs. 

​To clearly illustrate the difference between a propelled vehicle and an ocean glider, I have included ocean gliders as a subset of UUVs (NOT as examples of AUVs). It is also debatable whether ocean gliders are completely autonomous as they are reliant on water motion and are not as capable as propelled vehicles in reaching planned destinations - leading some experts (see [7]) to classify them separately from AUVs. Perhaps it is more appropriate to compare gliders to dinoflagellates, (mostly) marine plankton, who, due to their swimming and eating habits, are considered both phytoplankton and zooplankton [8].​​