Perhaps not the most in depth, but here is what is on bloomilk

attack of opportunity

A single, immediate attack against an adjacent enemy who moves. If an enemy moves out of a square adjacent to a character, that character can make an attack of opportunity against that enemy. Attacks of opportunity do not use the targeting rules. A character can’t use a special ability or Force power that replaces attacks instead of an attack of opportunity.

on bloomilk

Attacks of Opportunity

If an enemy moves out of a square adjacent to a character, that character can make a single, immediate
attack against that enemy (even if the character has already activated this round). This is called an attack
of opportunity.

One Per Turn: There is no limit to the number of attacks of opportunity a character can make in a round,
but it can make only one during a given character’s turn.

Attack is Optional: A character does not have to make an attack of opportunity when one is available.

Timing: A character makes an attack of opportunity in response to an enemy’s movement. The attack takes
place when the enemy is about to leave the adjacent square, but before it actually does. Pause the movement
and make the attack; if the enemy is not defeated, it continues moving.

Single Attack: An attack of opportunity is a single attack. Even characters with special abilities that allow them
to make additional attacks can make only one attack of opportunity. (Twin still happens - swine)

Walls: Two characters are adjacent only if they have line of sight to each other. If two characters are on
opposite sides of a wall, they aren’t adjacent, so neither can make attacks of opportunity against the other.

“Replaces Attacks”: Some special abilities take the place of attacks. They cannot be used in place of
attacks of opportunity.

I think he's probably looking for a glossary for Armed and Operational.