Prone

The character is lying on the ground. A prone attacker has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A prone defender gains a +4 bonus to Armor Class against ranged attacks, but takes a –4 penalty to AC against melee attacks.

Standing up is a move-equivalent action that provokes an attack of opportunity. Dropping prone is a free action.  While prone, you can crawl 5 feet as a move action.

Marius, Scholai Armsmaster Says: In certain battlefields--across rivers, over chasms--a battle can be fought entirely at range.  Drop prone to seize a quick advantage in such skirmishes. Some enemies will trip you with every attack.  In such situations, it is often better to just fight on the ground rather than give them free attacks as you struggle to your feet only to be knocked down again.