Under Improved Land 
A piece of land which has been improved, but not to the full extent of its potential.

Unencumbered Property
Any property which has no outstanding claims or liens against it.

Uniform Standards Of Professional Appraisal Practice (USPAP)  
Developed in 1986 by the Ad Hoc Committee on Uniform Standards and copyrighted in 1987 by The Appraisal Foundation, USPAP forms the guidelines followed by every licensed and certified real estate appraiser in the United States. The purpose of these Standards is to establish requirements for professional appraisal practice, which includes appraisal, appraisal review, and appraisal consulting. The intent of these Standards is to promote and maintain a high level of public trust in professional appraisal practice.

Unique visitors
A count of individual users who have accessed your web site. It should be noted that the "user session" metric does not yield an accurate unique visitor count, as multiple user sessions can be generated by one unique visitor.

Unix
An operating system designed to be used by more than one person at a time. It is the most common operating system for servers on the Internet.

Unlocked Domain
An unlocked domain name can be transferred to another registrar at any time.

Upload
The process of transferring files from your computer to a computer on the Internet.

URL
Used interchangeably with web address. Acronym stands for Uniform Resource Locator. URLs can specify the location of a web page, an email address, or a file on an FTP server, among other things.

Useful Life
The span of time over which a property can be used or can provide benefits to its owner.

User agent
The name of the browser/spider that is currently visiting a page.