The Collaborative International Dictionary of English v.0.48:

Beef \Beef\ (b[=e]f), n. [OE. boef, befe, beef, OF. boef, buef,
   F. b[oe]ef, fr. L. bos, bovis, ox; akin to Gr. boy^s, Skr.
   g[=o] cow, and E. cow. See 2d Cow.]
   1. An animal of the genus Bos, especially the common
      species, Bos taurus, including the bull, cow, and ox, in
      their full grown state; esp., an ox or cow fattened for
      food.

   Note: [In this, which is the original sense, the word has a
         plural, beeves (b[=e]vz).]
         [1913 Webster]

               A herd of beeves, fair oxen and fair kine.
                                                  --Milton.
         [1913 Webster]

   2. The flesh of an ox, or cow, or of any adult bovine animal,
      when slaughtered for food.

   Note: [In this sense, the word has no plural.] "Great meals
         of beef." --Shak.
         [1913 Webster]

   3. Applied colloquially to human flesh.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Bos taurus
    n 1: domesticated bovine animals as a group regardless of sex or
         age; "so many head of cattle"; "wait till the cows come
         home"; "seven thin and ill-favored kine"- Bible; "a team of
         oxen" [syn: cattle, cows, kine, oxen, Bos taurus]