The Collaborative International Dictionary of English v.0.48:

algae \algae\ n.
   plural of alga.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Alga \Al"ga\, n.; pl. Alg[ae] or algae. [L., seaweed.]
   (Bot.)
   A kind of seaweed; pl. the class of cellular cryptogamic
   plants which includes the black, red, and green seaweeds, as
   kelp, dulse, sea lettuce, also marine and fresh water
   conferv[ae], etc. The algae are primitive
   chlorophyll-containing mainly aquatic eukaryotic organisms
   lacking true stems and roots and leaves.
   [1913 Webster + WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Cryptogamia \Cryp`to*ga"mi*a\ (kr?p`t?-g?"m?-?), n.; pl.
   Cryptogami[ae] (-?). [NL., fr. Gr. krypto`s hidden, secret
   + ga`mos marriage.] (Bot.)
   The series or division of flowerless plants, or those never
   having true stamens and pistils, but propagated by spores of
   various kinds.
   [1913 Webster]

   Note: The subdivisions have been variously arranged. The
         following arrangement recognizes four classes: -- I.
         Pteridophyta, or Vascular Acrogens.} These include
         Ferns, Equiseta or Scouring rushes, Lycopodiace[ae]
         or Club mosses, Selaginelle[ae], and several other
         smaller orders. Here belonged also the extinct coal
         plants called Lepidodendron, Sigillaria, and
         Calamites. II. Bryophita, or Cellular Acrogens.
         These include Musci, or Mosses, Hepatic[ae], or
         Scale mosses and Liverworts, and possibly
         Charace[ae], the Stoneworts. III. Alg[ae], which
         are divided into Floride[ae], the Red Seaweeds, and
         the orders Dictyote[ae], Oospore[ae],
         Zoospore[ae], Conjugat[ae], Diatomace[ae], and
         Cryptophyce[ae]. IV. Fungi. The molds, mildews,
         mushrooms, puffballs, etc., which are variously grouped
         into several subclasses and many orders. The Lichenes
         or Lichens are now considered to be of a mixed nature,
         each plant partly a Fungus and partly an Alga.
         [1913 Webster] Cryptogamic
         Cryptogamian

WordNet (r) 3.0 (2006):

algae
    n 1: primitive chlorophyll-containing mainly aquatic eukaryotic
         organisms lacking true stems and roots and leaves [syn:
         alga, algae]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "algae":
   Ectocarpales, Iceland moss, Irish moss, Phaeophyceae, autophyte,
   bean, bracken, brown algae, climber, conferva, confervoid, creeper,
   diatom, dulse, fern, fruits and vegetables, fucoid, fucus, fungi,
   fungus, grapevine, green algae, gulfweed, herb, heterophyte, ivy,
   kelp, legume, lentil, liana, lichen, liverwort, mold, molds, moss,
   mushroom, parasite, parasitic plant, pea, perthophyte,
   phytoplankton, plankton, planktonic algae, plant families,
   pond scum, puffball, pulse, red algae, reindeer moss, rockweed,
   rust, saprophyte, sargasso, sargassum, scum, sea lentil, sea moss,
   sea wrack, seaweed, smut, succulent, thallogens, toadstool, vetch,
   vine, wort, wrack