Definition from Wiktionary, the free dictionary
Alternative forms 
- unlesse (obsolete)
- Except on a specified condition; if not.
- I’m leaving unless I get a pay rise.
- 1971, Dr. Seuss, The Lorax:
- Unless someone like you...cares a whole awful lot...nothing is going to get better...It's not.
- 1839, Denison Olmsted, A Compendium of Astronomy Page 95
- Secondly, When a body is once in motion it will continue to move forever, unless something stops it. When a ball is struck on the surface of the earth, the friction of the earth and the resistance of the air soon stop its motion.
- (except on a condition): if
Derived terms 
except on a specified condition