Unarmed strike

SRD 3.5 Unarmed strikes
An unarmed attack or unarmed strike is striking for damage with punches, kicks, and head butts is much like attacking with a melee weapon, but with the following changes.

Attacks of opportunity
Attacking unarmed provokes an attack of opportunity from the character you attack, provided she is armed. The attack of opportunity comes before your attack. An unarmed attack does not provoke attacks of opportunity from other foes nor does it provoke an attack of opportunity from an unarmed foe.

An unarmed character can’t take attacks of opportunity (but see “Armed” Unarmed Attacks, below).

"Armed” Unarmed Attacks
In the following cases a character’s or creature’s unarmed attack counts as an armed attack.
 * A monk
 * A character with the Improved unarmed strike feat
 * A spellcaster delivering a touch attack spell
 * A creature with natural physical weapons

Note that being armed counts for both offense and defense (the character can make attacks of opportunity)

Unarmed Strike Damage
The damage dealt by an unarmed strike depends on the size of the character as follows:
 * Small size: 1d2 plus Strength modifier
 * Medium size: 1d3 plus Strength modifier
 * Large size: 1d4 plus Strength modifier

All damage from unarmed strikes is nonlethal damage.

Unarmed strikes count as light weapons (for purposes of two-weapon attack penalties and so on).

Dealing Lethal Damage
You can specify that your unarmed strike will deal lethal damage before you make your attack roll, but you take a –4 penalty on your attack roll. If you have the Improved Unarmed Strike feat, you can deal lethal damage with an unarmed strike without taking a penalty on the attack roll.