A fanterm used to describe the yaoi(boyxboy) pairing of Alfred F. Jones and Arthur Kirkland, from the anime series Axis Powers Hetalia. In the series, countries are represented as "human" nations, as Alfred represents the United States of America and Arthur represents The United Kingdom of Great Britian and Northern Ireland, or simply just England.

The "US" part stands for Alfred, and the "UK" part stands for Arthur.

Although it is usually seen as "USUK," there is also, "UKUS." Occasionally, the order of the names makes no difference, but most of the time, it does imply who is "seme" and who is "uke" in the relationship. When "UKUS" is seen, it usually means that Arthur, or England, is being referred to as the "seme," leaving Alfred, or America, the "uke." However, "USUK" is more common.
> "Ooh, that was the cutest USUK [fanfic] ever!"
> "USUK is my favorite [Hetalia] pairing!"