Contrary to a coming out, where people decide for themselves when and to whom they disclose their identity (e.g. as lesbian, gay, bisexual, trans or inter), the term ‘outing’ refers to a situation in which the disclosure of this information takes place without their consent.
Outings can happen long after a person’s first coming out. Take the example of a trans man who is perceived as a cis man in his everyday life. As he does not talk about it, his friends and acquaintances do not know that he is trans at all and, maybe, he does not want to be seen as trans. Then, if an old acquaintance mentions that he is trans in this new context he is being outed.