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 purposefully to insult cis people for not obeying gender roles. 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.