A "nightcap" is an alcoholic beverage consumed after an evening out, or prior to bed once at home. A group of friends may agree to have a nightcap, or final drink, at the end of an evening together. A couple may have a nightcap to conclude a date.
A nightcap is not a specific drink, though wine is often consumed. Some individuals have a stronger alcoholic beverage as a nightcap to help them get to sleep. The old English meaning of the term was more targeted as a bedtime drink. After a date, an invitation for a "nightcap" is a common way of inviting a date into the home.