Jefft is the name taken by an extra-dimensional entity which Zim and Dib encounter in Issue 32 of the Invader Zim comic series.
Summary[]
During the National Physical Phitness challenge at the Skool, Zim and Dib each become determined to win first place, Zim so that he can meet President Man and disintegrate his head, and Dib just so that he can beat Zim.
Each is so determined, in fact, that they decide to cheat, with Zim creating a muscle suit from Vortian technology and Dib creating a mass of artificial muscles with his father's 3D flesh printer. This leaves them evenly matched during most of the challenge's exercises.
During the final exercise, sit-ups, Zim and Dib are completely matched, and neither is willing to give up. Due to this sheer determination, and certain aspects of Zim's suit bending space and time, the pair surpass an infinite number of sit-ups. This causes them to transcend outside of the multiverse, where they find themselves in a void surrounded by giant floating ab muscles. One of these approaches them, identifying itself first as "the Abs of Time and Space" and then, when both Zim and Dib refuse to call it that, as "Jefft" ("Jeff" with a "T" added on the end).
Jefft explains that it and the other Abs represent various universes throughout the multiverse, and also warns that Zim and Dib's endless sit-ups threaten to spread throughout the multiverse. It begs them to stop before this happens, offering to send them to the universe of their choice in exchange for them doing so.
Zim and Dib both consider Jefft's offer, but to its dismay, they both refuse to let the other win and continue their sit-ups, soon being multiplied across every universe in the multiverse.