Unique Visitor (or UV) is a web term designed to make website measurement and advertiser interactions more closely resemble a true count of individual persons. This is useful for websites, ad networks, and advertisers, as a proxy for reach and is a method of arriving at a common understanding of the available audience available with a given publisher. By having a concept of uniqueness, one can then look at repeat exposures and content consumption in terms of frequency (eg Average Page Views per Unique Visitor, and the concept of Frequency Capping in a display media buy).
Uniqueness can be determined in a number of ways, with varying degrees of discrepancy. One of the earlier and less reliable methods was to count unique IP addresses, although this tends to undercount uniqueness due to corporations and ISPs sharing IP banks. A more common method is to place a Cookie on each users browser, and then look for the presence of that cookie and tally content and advertising consumption against it. This tends to be more universally accepted, but has flaws due to cookie deletion by users and the fact that a shared computer may have several unique individuals counted as one Unique Visitor.
Comscore publishes monthly data which ranks websites and networks based on Unique Visitors and effective Reach. This data, and data published by Neilsen, tend to be the accepted currencies for audience size, although it is important to note that both build on an already imperfect method of calculating uniqueness with the imperfect extrapolation of sample panels.