Source : Webster's Revised Unabridged Dictionary (1913)

Subnormal \Sub*nor"mal\, n. (Geom.)
   That part of the axis of a curved line which is intercepted
   between the ordinate and the normal.

Source : WordNet®

subnormal
     n : a person of less than normal intelligence

subnormal
     adj : below normal or average; "after the floods the harvests were
           subnormal"; "subnormal intelligence"