Definition from Wiktionary, the free dictionary
From Hebrew ראש השנה (rosh-hashaná).
Proper noun 
- The Jewish holiday marking the start of the liturgical year, taking place on the first two days of Tishrei, 163 days after the first day of Passover.
- (Jewish holiday): Jewish New Year
Rosh Hashanah (plural Rosh Hashanahs)
- A specific occurrence of this holiday.