The Collaborative International Dictionary of English v.0.48:

Myriapoda \Myr`i*ap"o*da\, n. pl. [NL., fr. Gr. ? numberless +
   -poda.] (Zool.)
   A class, or subclass, of arthropods, related to the hexapod
   insects, from which they differ in having the body made up of
   numerous similar segments, nearly all of which bear true
   jointed legs. They have one pair of antennae, three pairs of
   mouth organs, and numerous tracheae, similar to those of true
   insects. The larvae, when first hatched, often have but three
   pairs of legs. See Centiped, Galleyworm, Milliped.
   [1913 Webster]

   Note: The existing Myriapoda are divided into three orders:
         Chilopoda, Chilognatha or Diplopoda, and
         Pauropoda (see these words in the Vocabulary). Large
         fossil species (very different from any living forms)
         are found in the Carboniferous formation.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Myriapoda
    n 1: arthropods having the body composed of numerous double
         somites each with two pairs of legs: millipedes [syn:
         Diplopoda, class Diplopoda, Myriapoda, class
         Myriapoda]