Potion (ポーション [potion] in Japanese) is the standard healing item in every Final Fantasy that includes such items. It restores HP.
See also Hi-Potion, X-Potion.
Final Fantasy I
Buy: 40 gil (sell: ? gil)
Shop: Cornelia, Elfheim, Pravoka
Drop: Black Goblin, Clay Golem, Crazy Horse, Goblin Guard, Sahagin
Description: Restores 50 HP
Final Fantasy II
Buy: 30 gil (sell: 15 gil)
Description: Restores a small amount of HP
Final Fantasy III

Buy: 50 gil (sell: ? gil)
Shop: Ur, Kazus, Kanaan, Tozus, Vikings' Cove, Village of the Ancients, Dwarven Hollows, Amur, Invincible
Drop: (numerous)
Steal: (numerous)
Treasure: Altar Cave, Ur, Kazus, Sasune Castle East Tower, Sealed Cave, Canaan
Type: Consumable
Effect: Restores 50 HP
Final Fantasy IV
Use: restores 100 HP
Buy: 30 gil (Sell: 15 gil)
Final Fantasy V
Use: Restores 50 HP, damages undead
Buy: 40 gil (Sell: 20 gil)
Drop: many enemies
Obtain: many chests
Final Fantasy VI
Use: restores 50 HP
Buy: ? gil (Sell: ? gil)
Final Fantasy VII

Use: restores 100 HP
Buy: 50 gil (sell: ? gil)
Shop: Below Junon, Bone Village, Cargo Boat, Cosmo Canyon, Costa Del Sol, Fort Condor, Gold Saucer, Gongaga, Junon, Kalm, Mideel, Nibelheim, Noth Corel, Rocket Town, Sector 5, Sector 7, Shinra Building, Temple of the Ancients, Wall Market
Final Fantasy IX

Buy: 50 gil (sell: 25 gil)
Shop: Alexandria (item shop (disc 1+3)), Priva Vista, North Gate] (Melda Arch shop), South Gate (Dali Gate shop, Bohden Arch shop, Summit shop, Vega item shop), Dali (item shop), Lindblum (Alice's shop, Dragon's Gate merchant, Dragoos' shop (disc 3)), Burmecia (Atlas's mogshop), Treno (slum item shop), Cleyra (Star Maiden Nina's shop), Fossil Roo (Mogki's mogshop, Treasure Hunter's shop, Kuppo's mogshop), Conde Petie (Wendy Grocer's shop), Black Mage Village (item shop), Madain Sari (Morrison's shop), Esto Gaza (equipment shop (before Desert Palace)), Hilda Garde 1, Mt Gulug (Mogtaka's mogshop), Daguerreo (item shop), Ipsen's Castle (Kumool's mogshop)
Drop: (various enemies)
Steal: (various enemies)
Treasure: (various treasures)
Obtain: from Mu, dig up during Chocobo Hot & Cold, dig up 5 on the world map with the Chocograph 'Between Mountains', 8 with 'Bird's-Eye Lagoon', 37 with 'Faraway Lagoon' and 33 with 'Fairy Island', use Dead Pepper for 50 on ocean bubbles between Lost and Outer continents
Use: Opal
Description: Restores 150 HP. The effect decreases if used in the field
Other: restores 100 HP on the field
Final Fantasy X
Buy: ? gil (sell: ? gil)
Obtain: (multiple ways)
Equip effect: -, Aeon effect: -
Description: Restores 200 HP to one party member
Other: deals 200 damage when used on undead
Final Fantasy XII
Use: restores 100 HP for target
Buy: 70 gil (sell: 35 gil)
Other: affected by Potion Lore (10%-15%-25%), Pheasant Netsuke, Nihopalaoa
Final Fantasy XIII-2
Use: restores ? HP
Buy: ? gil (sell: ? gil)
Obtain: various spheres, improved Moogle Throw
Description: Restores a small amount of HP to all allies
Final Fantasy XIV

Use: HP +40% (max 100 HP)
Buy: - (sell: 56 gil)
Craft: ALC 8 x6 - Water Shard x2, Yellow Ginseng, Tinolqa Mistletoe, Distilled Water
Type: Potion, Stack: 99
Description: This weak concoction instantly restores a few HP.
Final Fantasy Tactics
Use: restore 30 HP, horizontal range 4 (Chemist ability Item, 30 JP)
Buy: 50 gil (sell: 25 gil)
Shop: (available from the start of the game) everywhere
Description: Medicine that restores HP. Deals damage when used against the undead.
Ring of Fates
Buy: 10 gil (sell: 5 gil)
Drop: many, many enemies
Treasure: many, many chests and barrels
Other: recovers 150 HP for any player within the AoE
Type-0

Buy: 100 gil (sell: 10 gil)
Shop: Underling, Multi-player menu
Treasure: missions 1-1, 2-3, 3-1, Northern Tunnel, Genbu Underpass
Reward: guests
Obtain: default item at game start, turn in no-wing tags (1-9 at a time)
Description: Restores up to 50% HP for one party member.
Airborne Brigade

Shop: -
Obtain: various ranking/login rewards
Description: Restores a quarter of LP
Other: usage limit, up to 5 per day
Category: Item