Wikipedia: " A stereotype can be a conventional and oversimplified conception, opinion, or image, based on the assumption that there are attributes that members of the other group hold in common. "
It's like over-generalizing.
For example:
Men: tough, unfeeling, insensitive, combative, the owner or ruler of the home.
Women: the nurturers, caregivers, demure, polite, and the family homemaker.
