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]