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. What counts as a "close connection" can vary between demisexuals. 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 is established. Being sexually attracted to one's close friends does not necessarily mean that one is demisexual. An already existing relationship is the only way a demisexual person would be able to experience sexual attraction.
Additionally, it is important to note that demisexuals are not simply choosing to abstain from sexual activity until they know the person better. Allosexuals are capable of feeling sexual attraction to people they do not know very well, though they typically choose not to act on it. Demisexuals are incapable of feeling sexual attraction until a close emotional bond has been formed, though they may still engage in sexual activity with the assumption that attraction will develop at some point.
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.
The term demisexual was coined in 2006 on the AVEN forms by user sonofzeal, who described his experience of not being sexually attracted to people without first forming an emotional connection. Sonofzeal felt neither asexual nor allosexual and created the term to more accurately describe needing an emotional bond as a prerequisite to sexual attraction. The term started to gain widespread popularity in 2008.
It is unknown when the demisexual flag was created or by who, but it was presumably created after the asexual flag was created in 2010, as its use the same colors, which were based off the AVEN logo. In both the asexual and demisexual flags black and represents a lack of sexual attraction, and purple represents the ace-spec community. The grey represents greysexual, which demisexuality is included under. On the demisexual flag white represents sexual attraction.
The prefix demi- means half or partial. The prefix is also used in words such as demigod.