The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]