A NUN is a religious woman who has taken a vow of chastity and poverty, and is typically associated with the Catholic Church. An ADDICT is someone who is dependent on a substance or activity, and is unable to stop using it even when it causes harm. A USER is someone who regularly uses a particular product or service. A SISTER is a female sibling. Therefore, the term 'one with a habit' could refer to a NUN, an ADDICT, a USER, or a SISTER.