The Collaborative International Dictionary of English v.0.48:

Attention \At*ten"tion\, n. [L. attentio: cf. F. attention.]
   1. The act or state of attending or heeding; the application
      of the mind to any object of sense, representation, or
      thought; notice; exclusive or special consideration;
      earnest consideration, thought, or regard; obedient or
      affectionate heed; the supposed power or faculty of
      attending.
      [1913 Webster]

            They say the tongues of dying men
            Enforce attention like deep harmony.  --Shak.
      [1913 Webster]

   Note: Attention is consciousness and something more. It is
         consciousness voluntarily applied, under its law of
         limitations, to some determinate object; it is
         consciousness concentrated. --Sir W. Hamilton.
         [1913 Webster]

   2. An act of civility or courtesy; care for the comfort and
      pleasure of others; as, attentions paid to a stranger.
      [1913 Webster]

   To pay attention to, To pay one's attentions to, to be
      courteous or attentive to; to wait upon as a lover; to
      court.
      [1913 Webster]

   Syn: Care; heed; study; consideration; application;
        advertence; respect; regard.
        [1913 Webster]

WordNet (r) 3.0 (2006):

attention
    n 1: the process whereby a person concentrates on some features
         of the environment to the (relative) exclusion of others
         [syn: attention, attending] [ant: inattention]
    2: the work of providing treatment for or attending to someone
       or something; "no medical care was required"; "the old car
       needs constant attention" [syn: care, attention, aid,
       tending]
    3: a general interest that leads people to want to know more;
       "She was the center of attention"
    4: a courteous act indicating affection; "she tried to win his
       heart with her many attentions"
    5: the faculty or power of mental concentration; "keeping track
       of all the details requires your complete attention"
    6: a motionless erect stance with arms at the sides and feet
       together; assumed by military personnel during drill or
       review; "the troops stood at attention"

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "attention":
   NB, TLC, absorption, acclaim, act of courtesy, acuity, acuteness,
   agility, alertness, amenity, anticipation, application, assiduity,
   attentions, attentiveness, audibility, audience, audition,
   aural examination, aural sense, auscultation, awareness, benignity,
   brightness, bugging, care, carefulness, caution, circumspection,
   circumspectness, civility, cognizance, concentration, concern,
   conference, consciousness, considerateness, consideration,
   courtesy, debate, deference, deliberation, devoirs, diligence,
   distinction, duties, eager attention, ear, eavesdropping, egards,
   electronic surveillance, engrossment, examination by ear, favor,
   favorable attention, forethought, gallantry, graceful gesture,
   hearing, heed, heedfulness, heeding, homage, honor,
   hushed attention, immersion, industry, intentness, interview,
   keenness, kindliness, limelight, listening, listening in,
   loving care, mark, mind, mindfulness, nimbleness, note, notice,
   notoriety, observance, observation, polite act, preparedness,
   prominence, promptitude, promptness, publicity, punctuality,
   quickness, rapt attention, readiness, regard, regardfulness,
   regards, remark, respects, reverence, sedulity, sedulousness,
   sense of hearing, sensibility, sharpness, sleeplessness, smartness,
   solicitude, study, tender loving care, thoughtfulness, tryout,
   urbanity, wakefulness, wiretapping