“Back burners are people we are romantically and/or sexually interested in, who we’re not currently involved with, and with whom we keep in contact in the possibility that we might... more“Back burners are people we are romantically and/or sexually interested in, who we’re not currently involved with, and with whom we keep in contact in the possibility that we might someday connect romantically and/or sexually. People can have back burners even if they’re already in a romantic relationship with someone else. Also, a former romantic and/or sexual partner can still count as a back burner so long as we still desire a romantic and/or sexual connection with them.” (Dibble et al., 2015, p. 226)
https://www.psychologytoday.com/blog/love-online/201606/are-you-someone-s-back-burner-five-ways-tell less