The Collaborative International Dictionary of English v.0.48:

Insecta \In*sec"ta\, n. pl. [NL. See Insect.]
   1. (Zool.) One of the classes of Arthropoda, including those
      that have one pair of antenn[ae], three pairs of mouth
      organs, and breathe air by means of trache[ae], opening by
      spiracles along the sides of the body. In this sense it
      includes the Hexapoda, or six-legged insects and the
      Myriapoda, with numerous legs. See Insect, n.
      [1913 Webster]

   2. (Zool.) In a more restricted sense, the Hexapoda alone.
      See Hexapoda.
      [1913 Webster]

   3. (Zool.) In the most general sense, the Hexapoda,
      Myriapoda, and Arachnoidea, combined.
      [1913 Webster]

   Note: The typical Insecta, or hexapod insects, are divided
         into several orders, viz.: Hymenoptera, as the bees
         and ants; Diptera, as the common flies, gnats, and
         mosquitos; Aphaniptera, or fleas; Lepidoptera, or
         moths and butterflies; Neuroptera, as the ant-lions
         and hellgamite; Coleoptera, or beetles; Hemiptera,
         as bugs, lice, aphids; Orthoptera, as grasshoppers
         and cockroaches; Pseudoneuroptera, as the dragon
         flies and termites; Euplexoptera, or earwigs;
         Thysanura, as the springtails, podura, and lepisma.
         See these words in the Vocabulary.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Insecta
    n 1: insects; about five-sixths of all known animal species
         [syn: Insecta, class Insecta, Hexapoda, class
         Hexapoda]