The Collaborative International Dictionary of English v.0.48:

parallax second \par"al*lax sec"ond\, n.
   An annual parallax of one second of an arc; -- applied to
   celestial objects outside the solar system. It is used to
   measure the distance of an astronomical object from the
   Earth. A star which has an annual parallax of one second of
   an arc is considered to be one parsec (3.26 light years)
   distant from the earth. See parsec in the vocabulary.
   [PJC]