Source : Webster's Revised Unabridged Dictionary (1913)

Companion \Com*pan"ion\, n. [F. compagnon, OF. compaing, fr. an
   assumed LL. companio (cf. companium fellowship, a mess), fr.
   L. com- + panis bread. See {Pantry}.]
   1. One who accompanies or is in company with another for a
      longer or shorter period, either from choice or casually;
      one who is much in the company of, or is associated with,
      another or others; an associate; a comrade; a consort; a
      partner.

            The companions of his fall.           --Milton.

            The companion of fools shall smart for it. --Prov.
                                                  xiii. 20 (Rev.
                                                  Ver.).

            Here are your sons again; and I must lose Two of the
            sweetest companions in the world.     --Shak.

            A companion is one with whom we share our bread; a
            messmate.                             --Trench.

   2. A knight of the lowest rank in certain orders; as, a
      companion of the Bath.

   3. A fellow; -- in contempt. [Obs.] --Shak.

   4. [Cf. OSp. compa[~n]a an outhouse, office.] (Naut.)
      (a) A skylight on an upper deck with frames and sashes of
          various shapes, to admit light to a cabin or lower
          deck.
      (b) A wooden hood or penthouse covering the companion way;
          a companion hatch.

   {Companion hatch} (Naut.), a wooden porch over the entrance
      or staircase of the cabin.

   {Companion ladder} (Naut.), the ladder by which officers
      ascend to, or descend from, the quarter-deck. --Totten.

   {Companion way} (Naut.), a staircase leading to the cabin.

   {Knights companions}, in certain honorary orders, the members
      of the lowest grades as distinguished from knights
      commanders, knights grand cross, and the like.

   Syn: Associate; comrade; mate; compeer; partner; ally;
        confederate; coadjutor; accomplice.

Companion \Com*pan"ion\, v. t.
   1. To be a companion to; to attend on; to accompany. [R.]
      --Ruskin.

   2. To qualify as a companion; to make equal. [Obs.]

            Companion me with my mistress.        --Shak.

Source : WordNet®

companion
     n 1: a person who is frequently in the company of another;
          "drinking companions"; "comrades in arms" [syn: {comrade},
           {fellow}, {familiar}, {associate}]
     2: a traveler who accompanies you [syn: {fellow traveler}, {fellow
        traveller}]
     3: one paid to accompany or assist or live with another

companion
     v : be a companion to somebody [syn: {company}, {accompany}, {keep
         company}]