From Andrew Zimmerman Jones,
Your Guide to Physics.
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Definition: A graviton is a theoretical virtual particle which would mediate the force of gravity. It is proposed by various theories of quantum gravity. The graviton would support a quantum representation of gravity which would consolidate it with the other fundamental forces of physics, which are also mediated by virtual particles.

Gravitons have not been experimentally observed. The theoretical models that include them predict a massless particle of spin 2, which would make it a boson.