The Collaborative International Dictionary of English v.0.48:

Companion \Com*pan"ion\ (k[o^]m*p[a^]n"y[u^]n), 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.
      [1913 Webster]

            The companions of his fall.           --Milton.
      [1913 Webster]

            The companion of fools shall smart for it. --Prov.
                                                  xiii. 20 (Rev.
                                                  Ver.).
      [1913 Webster]

            Here are your sons again; and I must lose
            Two of the sweetest companions in the world. --Shak.
      [1913 Webster]

            A companion is one with whom we share our bread; a
            messmate.                             --Trench.
      [1913 Webster]

   2. A knight of the lowest rank in certain orders; as, a
      companion of the Bath.
      [1913 Webster]

   3. A fellow; -- in contempt. [Obs.] --Shak.
      [1913 Webster]

   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.
          [1913 Webster]

   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.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Companion \Com*pan"ion\, v. t.
   1. To be a companion to; to attend on; to accompany. [R.]
      --Ruskin.
      [1913 Webster]

   2. To qualify as a companion; to make equal. [Obs.]
      [1913 Webster]

            Companion me with my mistress.        --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

343 Moby Thesaurus words for "companion":
   Bayard, Don Quixote, Gawain, Lancelot, Ritter, Sidney, Sir Galahad,
   abigail, accompanier, accompanist, accompanyist, ace, adjunct,
   ally, alter ego, amah, amiable, amigo, analogon, analogue,
   associate, attend, attendant, au pair girl, ayah, bachelor, back,
   back stairs, balance rudder, banneret, baronet, batten, beak,
   beakhead, beam, bear, bedfellow, bedmate, betweenmaid, biddy,
   bilge keelson, bitt, board, bodyguard, bollard, bollard timber,
   bosom buddy, bow, bracket plate, bridge, bring, brother,
   brother-in-arms, buddy, bulkhead, bulwarks, bunkie, bunkmate,
   butty, caballero, cam cleat, camarade, capstan, carling, carry,
   casemate, cathead, cavalier, ceiling, centerboard, chamberfellow,
   chambermaid, chaperon, chaperone, chevalier, chum, classmate,
   cleat, close copy, close match, coadjutor, cognate, cohort,
   colleague, comate, companion, companion piece, companionable,
   companionway, company, compatriot, compeer, complacent, complement,
   comrade, concomitant, conduct, conductor, confederate, confrere,
   congenator, congener, conning tower, consociate, consort,
   consort with, convivial, convoy, cook, coordinate, copartner, copy,
   correlate, correlative, correspondent, counter, counterpart, crony,
   cutwater, daggerboard, davit, dead ringer, deadwood, double,
   duenna, duplicate, effigy, enchiridion, entrance, equivalent,
   escalier, escort, esquire, exact likeness, fantail, fellow,
   fellow member, fellow student, fellow traveler, femme de chambre,
   figurehead, fille de chambre, fire escape, flight of steps,
   forefoot, foresheets, foretop, frame, freeboard, futtock,
   gangplank, gangway, garboard strake, gentlewoman, girl,
   girl friend, good-natured, gossip, guard, gudgeon, guide, gunnel,
   gunwale, handbook, handmaid, handmaiden, hatch, hatchway, hawse,
   hawse timber, hawsehole, hawsepiece, hawsepipe, head, heel,
   hired girl, housemaid, icon, idol, image, incline, island, keel,
   keel and keelson, keelson, kevel, kindred spirit, kitchenmaid,
   knee, knight, knight bachelor, knight banneret, knight baronet,
   knight-errant, lady-help, lady-in-waiting, landing, landing stage,
   larboard, lee, lee side, leeward, like, likeness, live-in maid,
   live-out maid, living image, living picture, maid, maidservant,
   maintop, manual, match, mate, messmate, miniature, mirroring,
   mizzentop, model, near duplicate, nose, nursemaid, obverse,
   old crony, paddle wheel, pal, parallel, pard, pardner, parlormaid,
   partner, pendant, perron, photograph, picture, pintle, planking,
   playfellow, playmate, poop, port, porthole, portrait, portside,
   post, propeller, prow, pulpit, rail, ramp, reciprocal,
   reference book, reflection, resemblance, rib, roommate, rubbing,
   rubrail, rudder, rudderpost, rudderstock, run, running mate,
   safe-conduct, schoolfellow, schoolmate, scullery maid, scupper,
   scuttle, scuttlebutt, second self, semblance, servant girl,
   servitress, shadow, shaft tunnel, sheave hole, sheer strake,
   sheets, shelf, shelfpiece, shepherd, shipmate, side partner,
   sidekick, similitude, simulacrum, sister, skeg, snorkel, sociable,
   soubrette, soul mate, spiral staircase, spit and image,
   spitting image, squire, staircase, stairs, stairway, stanchion,
   starboard, stem, stepping-stones, steps, stern, stile, strake,
   such, suchlike, superstructure, swain, tail end, tally, teammate,
   the like of, the likes of, tiller, trace, tracing, transom,
   treads and risers, truck, tweeny, twin, upstairs maid, usher,
   very image, very picture, waiting maid, waterline, waterway,
   weather, weather side, weatherboard, wench, wheel, winch, windlass,
   windward, workfellow, yokefellow, yokemate



Bouvier's Law Dictionary, Revised 6th Ed (1856):

COMPANION, dom. rel. By 5 Edw. III., st. 5, c. 2, Sec. 1, it is declared to
be high treason in any one who "doth compass or imagine the death of our
lord the king, or our lady his companion," &c. See 2 Inst. 8, 9; 1 H. H. P.
C. 124.