In the expression $x^r$, the number $r$ is known as the exponent of $x$, particularly for $r \in \R$.
Also known as
The exponent is also called the index (plural indices).
- 1997: Donald E. Knuth: The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.) ... (previous) ... (next): $\S 1.2.2$: Numbers, Powers, and Logarithms: $(4)$