What it is
Offensive output (or offensive ability, attack power, offense [in the sense of a specific amount of damage, as opposed to the act of attacking or damage ability per time (throughput)], etc.) is one of a character’s properties. It is the capacity of the power behind all of a character’s moves, the strength of the ability to attack imbued with mobility, etc.
The lack thereof is sometimes compared to a “mosquito” or “squirt gun” in Japanese. If there is a common English term for this, my not knowing it is probably for the better.
In terms of numbers, the offense is one factor that goes into calculating actual damage done by a particular move (other relevant factors including correction and defensive ability).
When the button portion of a command for a move makes two or more different kinds of that move, it will generally make the differences in weak/strong versions have variations in the output.
The amount of damage given over a period of time (rather than at a moment), due to combos and a character’s ability to pour it on and such is not truly output, but throughput.