Zero-G, or Zero Gravity, is the condition of not being affected by gravity different than one’s immediate surroundings. In orbit around a planet or in deep space, any object not currently accelerating under its own engine power can be said to be in zero-g. Also sometimes spelled “zero-gee”.
Without gravity to hold one to the floor, one cannot simply walk down a hallway or sit in a chair. Instead, one floats down hallways, swims through the air, crawls along handholds built into the walls or ceiling, and must strap in to sit in a chair or lie on a bunk. As many older warships (such as the Archer-class) and nearly all small civilian ships lack artificial gravity systems, travel in space often involves prolonged periods of zero-g.
Prolonged exposure to low or zero gravity is dangerous for human beings and most other sentients. However, in the mid-21st century, early Earth Fleet scientists developed medications called Grav Pills which can prevent these negative consequences if taken regularly, and of course all Earth Fleet and other military and civilian personnel who spend prolonged periods in space or in zero-g are prescribed courses of these drugs.