Prone
If you are lying on the ground, you are prone. When prone, you take a –4 penalty to melee attack rolls. You also gain a +4 bonus to your Armor Class against ranged attacks but take a –4 penalty to your AC against melee attacks. If you are knocked down in zero gravity, you’re off-kilter (see page 276) instead of prone.
Source
Core Rulebook pg. 254