synodic month

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. See lunar month.

Wiktionary

1. n. 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.
2. n. the variably cyclic period from one new moon until the next.

WordNet 3.0

1. n. the period between successive new moons (29.531 days)