Source : Webster's Revised Unabridged Dictionary (1913)

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, a_{0}, b_{1},
   c_{2}, x_{n}, have 0, 1, 2, and n as subindices.