Magic (魔法 [mahou] or 'magic' in Japanese) is one of the basic battle commands in Final Fantasies. Normally it lets you choose a spell you've learnt from a list.
Note that in many early games, this command was instead replaced with a more specific 'White Magic', 'Black Magic', etc. The list below is by no means a complete representation of FF games where magic is chosen from a complete list during battle.
Final Fantasy III
Use: Freelancer, White Mage, Black Mage, Red Mage, Knight, Scholar, Evoker, Devout, Magus, Summoner, Sage, Onion Knight
Effect: Enables you to use magic in battle
Other: note that you must teach the character magic via spell tomes first before any actual spells are present in this list
Final Fantasy VI
Use: (any)
Effect: Enables you to use magic in battle
Other: note that only a few characters learn magic naturally, the rest must first learn magic via magicite for this command to be available
Final Fantasy VII
Use: (any)
Effect: Enables you to use magic in battle
Other: note that for this command to become available, the character must equip materia that contains magic-type abilities
Final Fantasy VIII
AP Required: (learned by default)
GF: Quezacotl, Shiva, Ifrit, Siren, Brothers, Diablos, Carbuncle, Leviathan, Pandemona, Cerberus, Alexander, Doomtrain, Bahamut, Cactuar, Tonberry, Eden
Effect: Enables you to use magic in battle
Final Fantasy XI
Use: (when equipped with any job capable of using any type of magic)
Effect: Enables you to use magic in battle
Other: note that when selecting this command from the battle menu, it will open a further menu that divided magic by type, and only after that selecting one type of magic you can view the spell list
Other: note that selecting abilities would usually be done by macro rather than via the battle menu during actual gameplay
Final Fantasy XII
Use: (any)
Effect: Enables you to use magic in battle
Other: to use this command, the character must first obtain a license for at least one magic spell, and furthermore the player must be in possession of the corresponding spell scroll
Category: Ability