The Collaborative International Dictionary of English v.0.48:

Hemorrhoids \Hem"or*rhoids\, n. pl. [L. haemorrhoidae, pl., Gr.
   ?, sing., ? (sc. ?), pl., veins liable to discharge blood,
   hemorrhoids, fr. ? flowing with blood; a"i^ma blood + ? to
   flow: cf. F. h['e]morro["i]des, h['e]morrho["i]des. See
   Rheum.] (Med.)
   Livid and painful swellings formed by the dilation of the
   blood vessels around the margin of, or within, the anus, from
   which blood or mucus is occasionally discharged; piles;
   emerods.

   Usage: [The sing. hemorrhoid is rarely used.]
          [1913 Webster]

WordNet (r) 3.0 (2006):

hemorrhoid
    n 1: pain caused by venous swelling at or inside the anal
         sphincter [syn: hemorrhoid, haemorrhoid, piles]