The Collaborative International Dictionary of English v.0.48:

Breather \Breath"er\, n.
   1. One who breathes. Hence:
      (a) One who lives.
      (b) One who utters.
      (c) One who animates or inspires.
          [1913 Webster]

   2. That which puts one out of breath, as violent exercise.
      [Colloq.]
      [1913 Webster]

   3. a pause to catch one's breath, or for some other form of
      rest or refreshment; -- often used in the phrase

   to take a breather, i.e. to pause for refreshment.
      [Colloq.]
      [PJC]

   4. a vent in a container to allow equalization of internal
      and external pressure.
      [PJC]

   5. an air intake pipe to provide air to machinery or people
      submerged or otherwise sealed off from the outside.
      [PJC]

WordNet (r) 3.0 (2006):

breather
    n 1: a short respite [syn: breath, breather, breathing
         place, breathing space, breathing spell, breathing
         time]
    2: air passage provided by a retractable device containing
       intake and exhaust pipes; permits a submarine to stay
       submerged for extended periods of time [syn: snorkel,
       schnorkel, schnorchel, snorkel breather, breather]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "breather":
   athletics, blow, break, breath, breathing place, breathing space,
   breathing spell, breathing time, calisthenics, cigarette break,
   cocktail hour, coffee break, constitutional, daily dozen, downtime,
   drill, enforced respite, exercise, exercising, gymnastic exercises,
   gymnastics, halt, happy hour, interlude, intermission, isometrics,
   letup, lull, pause, physical education, physical jerks, practice,
   recess, respite, rest, setting-up exercises, spell, stay, stretch,
   surcease, suspension, tea break, ten, time out, workout, yoga