A unicorn is someone who is unattainable, a person who possesses all of the characteristics that are most desirable to you in a partner, the person who has the potential to be the end all, be all of your world.
Understanding the unicorn term in dating is mostly about coming to terms with how difficult it can be to find one.
Sometimes, the unicorn is asked to leave the scene if the couple wants to spend intimate time together after fulfilling their experimental fantasies.