What is "subindex"

Subindex \Sub*in"dex\, n.; pl. Subindices. (Math.) A number or mark placed opposite the lower part of a letter or symbol to distinguish the symbol; thus, a0, b1, c2, xn, have 0, 1, 2, and n as subindices.

n. 1 A subsidiary index, or one that represents a sector of a larger one 2 (context mathematics English) A subscript