[@Blackmist16]
[quote=Steven Universe Wiki]Gems as a species are characterized by the gemstone each Gem has embedded somewhere on their bodies. Each Gem shown thus far is unique.Gems physical forms are projections from their gemstone[2], and Gems may change their physical form at will.Gems have numerous abilities, such as weapon-summoning, fusion....
"Fusion Gems" are the product of two or more Gems who fuse together, creating an entirely separate entity. Fusions are commonly formed in states of emergencies and used as a last resort, except in cases like Garnet, where Ruby and Sapphire decided to stay fused forever as a sign of their love and bond. [/quote]

In summary, a fusion is where two gems [i]fuse[/i] together.