Coming out is a phrase common in the LGBT community that means "to recognize one's sexual orientation, gender identity, or sex identity, and to [become] open about it with oneself and with others." People with nonbinary gender identities have to come out if they are to be recognized as nonbinary. This is because in cultures that recognize only the gender binary, nonbinary people have only the options of being closeted (not "out" or open about one's gender identity) or stealth (living as one's chosen gender without others knowing that one is trans). There are some particular needs that are unique to the situation of coming out as nonbinary, as opposed to coming out as anything else. For example, the average person has an idea of what lesbians are, and knows that they exist in real life. Whatever other obstacles a lesbian woman might face when she comes out, she likely doesn't have to contend with those particular issues. However, the average person doesn't know what nonbinary people are, and doesn't have context for them existing in real life. Anyone who comes out as nonbinary needs to be prepared to explain what nonbinary gender is, and be prepared for the possibility that others might not accept it as a real gender identity.
When coming out as nonbinary to someone in writing, it can help to use template letters to figure out what to say, and how to say it well. See the page template letters - coming out for a collection of these.
↑ 1.01.1This quote is a snippet from an answer to the survey conducted in the year 2018. Note for editors: the text of the quote, as well as the name, age and gender identity of its author shouldn't be changed.