Adjunct means something added on, but not part of the whole. An adjunct professor is someone who is hired by a college to teach but isn't a full member of the faculty.
This is a word you can figure out by taking it apart. From ad- "to" and -junct "join" (think "junction"), you can see that this is about joining something to another. "During lunch, Tim always sat at the girls' lacrosse-team lunch table, and they joked that he was an adjunct member of the team."