The Collaborative International Dictionary of English v.0.48:

Lunation \Lu*na"tion\, n. [Cf. Lunated.]
   The period of a synodic revolution of the moon, or the time
   from one new moon to the next; varying in length, at
   different times, from about 291/4 to 295/6 days, the average
   length being 29 d., 12h., 44m., 2.9s.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lunation
    n 1: the period between successive new moons (29.531 days) [syn:
         lunar month, moon, lunation, synodic month]