synodic month

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a month of approximately 29.53 days, measured from a lunar phase until the return of that same phase. The long-term average duration is 29.530 588 days (29 d 12 h 44 min 2.8 s), or about 29 ½ days.
• noun the variably cyclic period from one new moon until the next.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the period between successive new moons (29.531 days)