A Transvestite

A person who sometimes wears clothes traditionally worn by and associated with the opposite sex; typically a male who cross-dresses occasionally by habit or personal choice.
(clinical psychology, psychiatry, pathology) A person, typically a heterosexual male, who compulsively seeks and derives paraphilic sexual arousal from cross-dressing, especially if the urges and behavior cause the patient distress or social impairment.

This term is relatively formal (Latinate); cross-dresser is more casual, but whereas the verb cross-dress is common, the verb transvest is quite rare.
Transvestite should not be confused with transgender (see that term for more); transvestites are often happy with their gender and have no desire to change their sex, but simply enjoy being able to cross-dress from time to time. When speaking of to or about an individual who identifies as transgender, the term transvestite is typically seen as derogatory.
The term should also not be confused with drag queen (“person who performs femininity”) or drag king (“person who performs masculinity”); those terms are specifically for performers.
The clinical definition is far more restrictive than the colloquial usage of the term, drawing a sharp distinction between a transvestite versus those who engage in other types of cross-dressing not associated with sexual arousal—such as a drag queen who cross-dresses to perform a role for entertainment purposes.