The definition can be ambiguous. It depends upon: type of food, goal(s) of the aquarist, age of fish, condition of fish.
Uneaten dead food causes a decrease in water quality, and so is an important consideration when using dry or frozen. The replies above seem to have sufficiently addressed dry food.
Live food, particularly types which can survive immersed for >24hrs, e.g. feeders, black worm, mosquito larvae, etc, can be provided in unlimited quantities to juveniles in grow-out tanks, or adult females being conditioned for breeding.
Limits are to be imposed upon adult males. This can't normally apply to keepers of community tanks, but many of us who keep Anabantoids often separate genders (the OP mentions Bettas).
If longevity is a consideration, then adult fish should be fed very sparingly (and the temperature decreased), e.g. only what can be consumed in 1 minute once a day.
The OP mentions "multiple Betta tanks." When I had >100 jars of Plakats I would feed live black worms once every 12 to 24 hours. I'd walk along the rows with a pipette and the container of worms, squirting more than each fish could eat into each jar. When I got to the last jar I'd go back to the first jar, and using the pipette I'd remove the uneaten worms. The whole process took ~20 minutes.
I notice a troubling trend in modern aquarium keepers, where the measure of welfare seems to be steeped solely in terms of survival: if the fishes live, things are good, if the fishes die, things are bad. It is an inappropriate position to take. [Nathan Hill in PFK]