Demisexual is a sexual orientation on the asexual spectrum defined as someone who does not experience sexual attraction until they have formed a deep emotional connection with someone. The connection can be romantic, platonic, or some other form of connection. Forming an emotional bond with someone does not mean that one is automatically attracted to said person, as it just means there's now a possibility for one to feel attraction.
It is important to distinguish between typical sexual attraction to friends and romantic partners, and demisexuality, which implies that there would be no sexual attraction to anyone unless a close relationship was to be established. The already existing relationship is the only way a demisexual person would be able to experience sexual attraction. Being sexually attracted to one's close friends does not necessarily mean that one is demisexual, for example.
Demisexual can be a sexual orientation on its own or can be combined with other orientations. For example, one could be demisexual and polysexual (demipolysexual), meaning that when one does experience sexual attraction they can experience it towards multiple genders, but not all genders.
Counterparts to demisexual include demiromantic, demiplatonic, and demialterous.
Etymology
The prefix demi- means half or partial. The prefix is also used in words such as demigod.