NOUN (1)

1. a person of less than normal intelligence;


ADJECTIVE (1)

1. below normal or average;
- Example: "after the floods the harvests were subnormal"
- Example: "subnormal intelligence"

The Collaborative International Dictionary of English v.0.48:

Subnormal \Sub*nor"mal\, n. (Geom.)
   That part of the axis of a curved line which is intercepted
   between the ordinate and the normal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subnormal
    adj 1: below normal or average; "after the floods the harvests
           were subnormal"; "subnormal intelligence"
    n 1: a person of less than normal intelligence

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "subnormal":
   aberrant, abnormal, amorphous, anomalistic, anomalous, arrested,
   babbling, backward, blithering, burbling, crackbrained, cracked,
   crazy, cretinistic, cretinous, deviative, different, dithering,
   divergent, driveling, drooling, eccentric, erratic, formless,
   half-baked, half-witted, heteroclite, heteromorphic, idiotic,
   imbecile, imbecilic, irregular, low, maundering,
   mentally defective, mentally deficient, mentally handicapped,
   mentally retarded, mongoloid, moronic, not all there, retarded,
   shapeless, simple, simpleminded, simpletonian, slobbering, stray,
   straying, subpar, unnatural, wandering