Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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