Broadly, distinctiveness is what makes something different, unique, and distinct from other things.
In psychology distinctiveness can have a few different meanings. In attribution theory, distinctiveness is when a behavior or action by an individual is judged by another to be common or unusual. This requires knowledge of the individual and their typical behaviors - this is called distinctiveness information.
Another concept using distinctiveness is contextual distinctiveness. This refers to the ability to better recall and remember events that are unusual, uncommon, and distinct. This also applies to stimuli - recall is better for stimuli that stand out and contrast against their environment.
Positive distinctiveness is a concept in Social Identity Theory and refers to when a group is made to appear in a more positive light typically being compared favorably to other groups. Their positive attributes are distinguished from other groups. This is common in sports teams.