Short for "semi-automatic."
1. (adj.) The characteristic, of a firearm, of being able to fire a single round for each pull of the trigger.
2. (n.) A firearm with this characteristic.
Commonly mistaken to mean fully automatic, which indicates a firearm most people would call a "machine gun," with which the user pulls the trigger a single time and holds it and the entire magazine is fired in succession, usually within a few seconds.
My Glock is a semi-auto with a 10-round magazine, so if I want to fire ten shots I have to pull the trigger ten times.