Unethical SEO
The use of unscrupulous optimisation techniques and can result in getting sites banned from the search engines.

Unique Visitors 
Unique visitors are a count of individual users who have accessed a 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.

URL (Uniform Resource Locator)
A Web Address. URLs can specify the location of a web page, an email address, or a file on an FTP server, among other things.

URL Rewrite 
A technique used to help make web site URLs more user and search engine friendly.
For example removing weird query strings generated by dynamic sites.

Usability
The ease a person browsing a web page can perform or complete any given task on a website. How user friendly a web site is.

User Agent
The name of the browser/spider that is currently visiting a page. For example, "Googlebot/2.1 (+http://www.google.com/bot.html)"

User Generated Content (UGC)
Content created and published by the end-users online. USG is comprised of videos, podcasts and posts on discussion groups, blogs, wiki's and social media sites.

Usenet 
Bulletin board network featuring thousands of newsgroups.

User Session
When an internet user accesses a web site for a length of time, then leaves. During a user session any number of pages may be accessed. A user session is considered finished once an arbitrarily period of inactivity, typically 30 minutes, is exceeded.