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]