Weightlessness is experience by people during free-fall. Although the term 'zero gravity' is often used as a synonym, weightlessness in orbit is not the result of gravity itself being eliminated or even reduced significantly (in fact, the acceleration towards earth due to gravity at an altitude of 100 km is only 3% less than at the earth's surface.) Weightlessness (roughly speaking) occurs when a body (e.g. a person) is: falling freely; in orbit; in outer space (far from a planet, star, or other massive body); in an airplane following a particular parabolic flight path (e.g. the "Vomit Comet"); or one of several other (even more unusual) frames of reference.