Undeliverables — See Nixies.

Unidentified sex — A group of names on a mailing list that contain men and women but do not have titles to set them apart. Mailers who get a strong response using a title select will often want to mail the unidentified sex portion as well.

Unit of sale — The amount paid for a product or service. This could be a fixed amount, as when each buyer has paid $25 for an item, or a blended rate reflecting a range of prices.

Universe — A total, such as the total number of names on a list, or a demographic total such as the total number in an age-group.

Updates — Regular additions of records to a database that keep files current.

Updating — The process of adding new records to a database either singly or in groups.

Usage history — A listing of previous mailers to a given list.

User profile — Information about an account-holder on a computer.