The definition of a traditional family is a social unit that consists of parents and children, whether living together or not. This definition is not immutable nor is it specific.
Family no longer refers to individuals in a group related by blood. Family now includes people who form close bonds. These can be friends, lovers, or a community of some kind. Even the idea of a traditional family containing a mother, father and children has changed. With the advent of gay marriage rights, traditional families could be two moms and children, or two dads and children. The idea of family is a social construct, therefore society can alter the definition as society grows and changes.