The Collaborative International Dictionary of English v.0.48:

Pinworm \Pin"worm`\, n. (Zool.)
   A small nematoid worm (Oxyurus vermicularis), which is
   parasitic chiefly in the rectum of man. It is most common in
   children and aged persons.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pinworm
    n 1: small threadlike worm infesting human intestines and rectum
         especially in children [syn: pinworm, threadworm,
         Enterobius vermicularis]