The Collaborative International Dictionary of English v.0.48:

Thespian \Thes"pi*an\, a. [From L. Thespis, Gr. ?, the founder
   of the Greek drama.]
   Of or pertaining to Thespis; hence, relating to the drama;
   dramatic; as, the Thespian art. -- n. An actor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thespian
    adj 1: of or relating to drama; "the movie director had thespian
           cooperation"
    n 1: a theatrical performer [syn: actor, histrion, player,
         thespian, role player]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "thespian":
   Roscius, actor, actor-proof, actress, all-star, antagonist,
   bad guy, ballet, balletic, barnstormer, character, character actor,
   character man, character woman, child actor, cinematic,
   cinematographic, diseur, diseuse, dramatic, dramatical, dramatizer,
   dramaturgic, feeder, film, filmic, foil, ham, hammy, heavy,
   histrio, histrion, histrionic, impersonator, ingenue, juvenile,
   legitimate, matinee idol, melodramatic, milked, mime, mimer, mimic,
   monodramatic, monologist, movie, mummer, operatic, overacted,
   overplayed, pantomime, pantomimist, performer, playactor, player,
   protean actor, reciter, scenic, soubrette, spectacular,
   stage performer, stage player, stagelike, stageworthy, stagy,
   starstruck, stellar, stooge, straight man, stroller,
   strolling player, theaterlike, theatrical, thrown away, trouper,
   underacted, underplayed, utility man, vaudevillian, villain