The most recently issued U.S. Treasury bond or note of a particular maturity. These are the opposite of "off-the-run treasuries". 

|||When quoting the price or yield of a Treasury, all market commentary refers to the on-the-run issue. The on-the-run bond or note is the most frequently traded Treasury security of its maturity. Because on-the-run issues are the most liquid, they typically are a little bit more expensive and, therefore, yield less than their off-the-run counterparts.