The Collaborative International Dictionary of English v.0.48:

Y \Y\ (w[imac]), n.; pl. Y's (w[imac]z) or Ys.
   Something shaped like the letter Y; a forked piece resembling
   in form the letter Y. Specifically:
   (a) One of the forked holders for supporting the telescope of
       a leveling instrument, or the axis of a theodolite; a
       wye.
   (b) A forked or bifurcated pipe fitting.
   (c) (Railroads) A portion of track consisting of two
       diverging tracks connected by a cross track.
       [1913 Webster]

   Y level (Surv.), an instrument for measuring differences of
      level by means of a telescope resting in Y's.

   Y moth (Zool.), a handsome European noctuid moth Plusia
      gamma) which has a bright, silvery mark, shaped like the
      letter Y, on each of the fore wings. Its larva, which is
      green with five dorsal white species, feeds on the
      cabbage, turnip, bean, etc. Called also gamma moth, and
      silver Y.
      [1913 Webster]