Wiktionary:Word of the day/2025/March 8

1. (arithmetic) The arithmetic operator consisting of repeated exponentiation, by analogy with exponentiation being repeated multiplication and multiplication being repeated addition, ${\displaystyle ^{b}a}$ denoting ${\displaystyle a}$ to the power of ${\displaystyle a}$ to the power of ${\displaystyle \ldots }$ to the power of ${\displaystyle a}$, in which ${\displaystyle a}$ appears ${\displaystyle b}$ times.

The English mathematician Reuben Goodstein, who coined the term, died on this day 40 years ago in 1985.