The theory that holds that we each have a sense of ourselves as a unique individual but that we also define ourselves based on our group memberships is called

Social identity theory.

Explanation:

Social Identity Theory (SIT): It was developed by Tajfel & Turner in 1979.

It is defined as an individual's perception of oneself of who they are in any group membership. Eventually, people tend to be in a group where they find their pride and self-esteem as being part of the group. These groups can be any of them like family, peers, and social class, etc.

There are three components of social identity theory:

1. Social categorization.

2. Social identification.

3. Social comparison.

Example:

1. Ethnic and religious identities.

2. Political identities.

3. Personal relationships.