The Collaborative International Dictionary of English v.0.48:

Protozoon \Pro`to*zo"["o]n\ (-[o^]n), n.; pl. Protozoa. [NL.]
   (Zool.)
      (a) One of the Protozoa.
      (b) A single zooid of a compound protozoan.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Protozoa \Pro`to*zo"a\, n. pl. [NL., fr. Gr. ? first + zo^,on an
   animal.] (Zool.)
   The lowest of the grand divisions of the animal kingdom.
   [1913 Webster]

   Note: The entire animal consists of a single cell which is
         variously modified; but in many species a number of
         these simple zooids are united together so as to form a
         compound body or organism, as in the Foraminifera and
         Vorticell[ae]. The reproduction takes place by fission,
         or by the breaking up of the contents of the body after
         encystment, each portion becoming a distinct animal, or
         in other ways, but never by true eggs. The principal
         divisions are Rhizopoda, Gregarin[ae], and Infusoria.
         See also Foraminifera, Heliozoa, Protoplasta,
         Radiolaria, Flagellata, Ciliata.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Protozoa
    n 1: in some classifications considered a superphylum or a
         subkingdom; comprises flagellates; ciliates; sporozoans;
         amoebas; foraminifers [syn: Protozoa, phylum Protozoa]