X in an equation (center)
X was a letter of the alphabet, often used to stand in for unknown variables in mathematical equations. (TNG: "The Nth Degree")
Crates with XXX printed on them were at Bela Okmyx's hideout. (TOS: "A Piece of the Action")
X was used as a variable in a solution to Fermat's last theorem. (TNG: "The Royale")