The Collaborative International Dictionary of English v.0.48:

Heliacal \He*li"a*cal\, a. [Gr. ? belonging to the sun, fr. ?
   the sun: cf. F. h['e]liaque.] (Astron.)
   Emerging from the light of the sun, or passing into it;
   rising or setting at the same, or nearly the same, time as
   the sun. --Sir T. Browne.
   [1913 Webster]

   Note: The heliacal rising of a star is when, after being in
         conjunction with the sun, and invisible, it emerges
         from the light so as to be visible in the morning
         before sunrising. On the contrary, the heliacal setting
         of a star is when the sun approaches conjunction so
         near as to render the star invisible.
         [1913 Webster]

WordNet (r) 3.0 (2006):

heliacal
    adj 1: pertaining to or near the sun; especially the first
           rising of a star after and last setting before its
           invisibility owing to its conjunction with the sun; "the
           heliacal rising of the Dog Star"; "the heliacal or Sothic
           year is determined by the heliacal rising of Sothis (the
           Egyptian name for the Dog Star)" [syn: heliacal,
           heliac]