The term Sensei is a “Shogo” or title.
The term “Sensei” refers to teacher or instructor. It is typically reserved for someone who has reached black belt level who is in a teaching capacity and often is reserved for those with 3rd dan, sandan, level and above who are actively teaching. The term is also used outside of the martial arts.
Many styles use the term as an honorific for all black belt holders whether or not they are in a teaching capacity.