Battery and charger for airsoft gun, or recharger, is a device that stores energy in a battery by running an electric current through it.
The charging protocol (how much voltage or current for how long, and what to do when charging is complete) depends on the size and type of the battery being charged.
Hilong Smart Airsoft Battery Charger is a rudimental AC input balance charger, having the characteristic of over-voltage protection, short circuit protection, over current protection, and reverse polarity protection. You can find batteries for airsoft guns with chargers, NIMH Battery Charger, and more here.