Inclusion is the act of including someone in something, like a school, club, or team.
If you are familiar with the word include — meaning to make someone or something a part of something larger — then inclusion should make sense. You might be happy at your inclusion on the school's volleyball team. The inclusion of an actor in a movie is exciting. When there's a new President, people wait and see who will earn inclusion in the cabinet. In biology, an inclusion body is a very small body found within another on the cellular level.