Definition of Rosh Chodesh in US English:

Rosh Chodesh

noun

Judaism
  • The observance of the appearance of the new moon, marking the beginning of the Jewish month; the first day of the Jewish month.

Origin

Mid 18th century. From post-biblical Hebrew rō'š ḥodeš first day of the month from biblical Hebrew rō'š beginning + ḥodeš month.