The Collaborative International Dictionary of English v.0.48:

Arachnida \A*rach"ni*da\, n. pl. [NL., fr. Gr. ? spider.]
   (Zool.)
   One of the classes of Arthropoda. See Illustration in
   Appendix.
   [1913 Webster]

   Note: They have four pairs of legs, no antenn[ae] nor wings,
         a pair of mandibles, and one pair of maxill[ae] or
         palpi. The head is usually consolidated with the
         thorax. The respiration is either by tranche[ae] or by
         pulmonary sacs, or by both. The class includes three
         principal orders: Araneina, or spiders;
         Arthrogastra, including scorpions, etc.; and
         Acarina, or mites and ticks.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Arachnida
    n 1: a large class of arthropods including spiders and ticks and
         scorpions and daddy longlegs; have four pairs of walking
         legs and no wings [syn: Arachnida, class Arachnida]