Misgendering describes any situation in which a person is referred to as, or implied to be, a gender other than their gender identity. This includes using the wrong form of gendered language, such as pronouns or titles, but can also include challenging someone's right to be in a gender-restricted area such as a public bathroom.
Misgendering can be accidental, which is usually a result of making assumptions based upon a person's gender expression, or it can be an intentional act of transphobia. Misgendering could also be used to insult cis people for not obeying gender roles, for example "Are you a girl? I heard you scream like one." Whether it was intentional or not, misgendering is often hurtful and can cause or exacerbate feelings of dysphoria.
Misgendering can be described as a microaggression - a small discriminatory act which, although so brief that the person responsible may be unaware they did anything wrong, occurs so often that it creates a constant negative atmosphere for the person experiencing it.