Sapiosexual is defined as when one finds intelligence sexually attractive or arousing, or finds intelligence to be the most attractive attribute in a person. Sapiosexual can sometimes be classified as an asexual spectrum identity, but it isn't always.
Often times sapiosexual is more accurately labeled as a preference, since sexual orientation refers to the genders one is attracted to. Some people use this label to make themselves sound smarter than others.
History
The term was allegedly coined by a LiveJournal user named wolfieboy, "while on too little sleep driving up from SF in the summer of '98."[1] In November of 2014 OKCupid expanded their list of sexual orientations and genders, including sapiosexual on the list[2], which is what made the mainstream public aware of the identity.
Controversy
Many people, rightfully, have pointed out that wanting a smart partner is a preference, not a sexual orientation. Additionally, desiring smart partners isn't a unique or non-normative preference. Most people will say that intelligence is quality they would want in a potential partner.
It has also been criticized for being ablest and classist, and limiting to the conversation around intelligence. In a Buzzfeed quiz titled "Are You Actually A Sapiosexual," one of the questions asks readers whether they're "repulsed by the idea of having sex with someone who had never gone to college, or had no interest in higher education,"[3] which seems to normalizing the idea of discriminating against those who don't have college degrees or explicitly academic aspirations.
There are many types of intelligence, such as inter-personal intelligence, or naturalist intelligence. However, the majority of the time sapiosexuals are only talking about pure academic intelligence, dismissing people who might not might not perform well in academic situations.