The Collaborative International Dictionary of English v.0.48:

Fungi \Fun"gi\ (f[u^]n"j[imac]), n. pl.; sing. fungus. (Biol.)
   A group of thallophytic plant-like organisms of low
   organization, destitute of chlorophyll, in which reproduction
   is mainly accomplished by means of asexual spores, which are
   produced in a great variety of ways, though sexual
   reproduction is known to occur in certain Phycomycetes, or
   so-called algal fungi. They include the molds, mildews,
   rusts, smuts, mushrooms, toadstools, puff balls, and the
   allies of each. In the two-kingdom classification system they
   were classed with the plants, but in the modern five-kingdom
   classification, they are not classed as plants, but are
   classed in their own separate kingdom fungi, which includes
   the phyla Zygomycota (including simple fungi such as bread
   molds), Ascomycota (including the yeasts), Basidiomycota
   (including the mushrooms, smuts, and rusts), and
   Deuteromycota (the fungi imperfecti). Some of the forms,
   such as the yeasts, appear as single-celled microorganisms,
   but all of the fungi are are eukaryotic, thus distinguishing
   them from the prokaryotic microorganisms of the kingdon
   Monera.
   [Webster 1913 Suppl. +PJC]

   Note: The Fungi appear to have originated by degeneration
         from various alg[ae], losing their chlorophyll on
         assuming a parasitic or saprophytic life. In an earlier
         classification they were divided into the subclasses
         Phycomycetes, the lower or algal fungi; the
         Mesomycetes, or intermediate fungi; and the
         Mycomycetes, or the higher fungi; by others into the
         Phycomycetes; the Ascomycetes, or sac-spore fungi;
         and the Basidiomycetes, or basidial-spore fungi.
         [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Fungus \Fun"gus\, n.; pl. L. Fungi, E. Funguses. [L., a
   mushroom; perh. akin to a doubtful Gr. ? sponge, for ?; if
   so, cf. E. sponge.]
   1. (Bot.) Any one of the Fungi, a large and very complex
      group of thallophytes of low organization, -- the molds,
      mildews, rusts, smuts, mushrooms, toadstools, puff balls,
      and the allies of each. See fungi.
      [1913 Webster]

   Note: The fungi are all destitute of chorophyll, and,
         therefore, to be supplied with elaborated nourishment,
         must live as saprophytes or parasites. They range in
         size from single microscopic cells to systems of
         entangled threads many feet in extent, which develop
         reproductive bodies as large as a man's head. The
         vegetative system consists of septate or rarely
         unseptate filaments called hyph[ae]; the aggregation of
         hyph[ae] into structures of more or less definite form
         is known as the mycelium. See Fungi, in the
         Supplement.
         [1913 Webster]

   2. (Med.) A spongy, morbid growth or granulation in animal
      bodies, as the proud flesh of wounds. --Hoblyn.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fungus
    n 1: an organism of the kingdom Fungi lacking chlorophyll and
         feeding on organic matter; ranging from unicellular or
         multicellular organisms to spore-bearing syncytia

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "fungus":
   adenovirus, aerobe, aerobic bacteria, amoeba, amphibian, anaerobe,
   anaerobic bacteria, angiosperm, annual, aquatic plant, bacillus,
   bacteria, bacterium, benign tumor, biennial, blast, blight,
   bread mold, bug, callosity, callus, cancer, canker, carcinoma,
   coccus, corn, cosmopolite, cutting, cyst, deciduous plant, dicot,
   dicotyledon, disease-producing microorganism, dry rot, echovirus,
   enterovirus, ephemeral, ergot, evergreen, excrescence, exotic,
   filterable virus, flowering plant, fungosity, gametophyte, germ,
   gram-negative bacteria, gram-positive bacteria, green mold, growth,
   gymnosperm, hydrophyte, intumescence, malignant growth,
   metastatic tumor, microbe, microorganism, mildew, mold, mole,
   monocot, monocotyl, morbid growth, moth, moth and rust, mushroom,
   must, neoplasm, nevus, nonfilterable virus, nonmalignant tumor,
   outgrowth, pathogen, perennial, pest, picornavirus, plant, polycot,
   polycotyl, polycotyledon, protozoa, protozoon, proud flesh,
   puffball, reovirus, rhinovirus, rickettsia, rot, rust, sarcoma,
   seed plant, seedling, slime mold, smut, spermatophyte, spirillum,
   spirochete, spore, sporophyte, staphylococcus, streptococcus,
   thallophyte, tinea, toadstool, triennial, truffle, trypanosome,
   tuckahoe, tumor, vascular plant, vegetable, verruca, verticillium,
   vibrio, virus, wart, water mold, weed, wen, worm, yeast