Demi-sexual is a sexual orientation where an individual is sexually attracted to someone only after forming an emotional connection with another person. Demi-sexuality is a spectrum because the kind of emotional connection a person requires depends on the individual. A part of the asexuality spectrum, demi-sexual folks can also have another sexual orientation where they define their attraction based on gender.
Sources:
https://lgbtq.unc.edu/resources/exploring-identities/asexuality-attraction-and-romantic-orientation/