A beta male is a man who has not been sexually active with his partner.
A beta sei is a woman who has been sexually and romantically involved with a beta male, but has not experienced sexual or romantic love.
The term beta male or beta seina is used to describe a man or woman who is not sexually and emotionally involved with his or her partner, but is willing to spend time and energy with the partner and is willing and able to take responsibility for the relationship.
A man or a woman will generally prefer to be referred to as a beta, but will not necessarily be identified as such.
The term beta club is also used to refer to a beta men and a beta seinen club.