Retal is short for retaliation, and refers to a special attack you are privledged with if someone attacks you first.
The normal rules of attack are suspended for a retal, in that you may attack the player who attacked you from ANY power rating, i.e. far above the normal 129% limit. Naturally, this stacks the deck in your favor.
However, the enemy empire must be out of damage protection to be attacked, and a "retal" expires after one week.