Source : Webster's Revised Unabridged Dictionary (1913)

Infusoria \In`fu*so"ri*a\, n. pl. [NL.; -- so called because
   found in infusions which are left exposed to the air for a
   time. See {Infuse}.] (Zo["o]l.)
   One of the classes of Protozoa, including a large number of
   species, all of minute size.

   Note: They are found in all seas, lakes, ponds, and streams,
         as well as in infusions of organic matter exposed to
         the air. They are distinguished by having vibrating
         lashes or cilia, with which they obtain their food and
         swim about.They are devided into the orders Flagellata,
         Ciliata, and Tentaculifera. See these words in the
         Vocabulary. Formely the term Infusoria was applied to
         all microscopic organisms found in water, including
         many minute plants, belonging to the diatoms, as well
         as minute animals belonging to various classes, as the
         Rotifera, which are worms; and the Rhizopoda, which
         constitute a distinct class of Protozoa. Fossil
         Infusoria are mostly the siliceous shells of diatoms;
         sometimes they are siliceous skeletons of Radiolaria,
         or the calcareous shells of Foraminifera.

{Protozoa}, including {Infusoria} and {Rhizopoda}. For
definitions, see these names in the Vocabulary.

Source : WordNet®

Infusoria
     n : in some recent classifications, coextensive with the
         Ciliata: minute organisms found in decomposing infusions
         of organic matter [syn: {subclass Infusoria}]