Speed Plus (not to be confused with the Materia of the same name), also known as Speed Source and Spd Up, is a recurring item in the series. It first appeared in Final Fantasy VII, and last appeared in the 20th Anniversary of Final Fantasy. It permanently increases a character's Speed stat by 1, and is thus a very rare item.
This item is called Speed and it has a potion icon before its name. It can be found in assorted treasure chests throughout the various worlds and increased a character's Agl stat by 3. It's also a one time use item as opposed to other consumable items which have more uses. Only only usable on humans or mutants.