Assumption: An assumption is something we take for granted or presuppose. All human thought and experience is based on assumptions. Assumptions can be unjustified or justified, depending upon whether we do or do not have good reasons for what we are assuming.

What we take for granted.

The following definition was taken from the official website for critical thinking:
http://www.criticalthinking.org/resources/k12/making-ct-intuitive.cfm