Asmall sac that lubricates the area around the elbow, knee, or shoulder joint where friction is most likely to occur is known as [one word]

Bursa - A small sac that lubricates the area around a joint where friction is most likely to occur 
ex. elbow, knee, or shoulder

The answer is atmosphere. there are four spheres of the earth: 1. geosphere (from greek word 'geo' = ground), 2. hydrosphere (from greek word 'hydro' = water), 3. biosphere (from greek word 'bio' = life), and 4. atmosphere (from greek word 'atmo' = air). a magnetosphere is a region of space around astronomical objects controlled by the magnetic field of that object. through the process of elimination, since lightning is present in the air, it can be found in the atmosphere.