noun
synodic month; lunar month
Technical term in astronomy; the period from one new moon to the next, about 29.5 days.
朔望月は約29.5日である。
A synodic month is about 29.5 days.
恒星月 is the sidereal month (about 27.3 days), measured relative to the fixed stars, while 朔望月 is measured relative to the sun and moon phases.
From 朔 (new moon) + 望 (full moon) + 月 (month). The term directly describes the cycle from new moon to full moon and back.