tetration
Tetration (noun) means a mathematical operation formed by repeating exponentiation, in the same way multiplication repeats addition, where a number is raised to a power of itself several times over.
Etymology
PIE word *kʷetwóres From tetra- (prefix meaning ‘four’) + (itera)tion, from the fact that tetration is in fourth place after addition, multiplication, and exponentiation. The word was coined by the English mathematician Reuben Goodstein (1912–1985).