One-Year Constant Maturity Treasury - 1-Year CMT

What Does One-Year Constant Maturity Treasury - 1-Year CMT Mean?
The interpolated one-year yield of the most recently auctioned four-, 13- and 26-week U.S. Treasury bills, plus the most recently auctioned 2-, 3-, 5- and 10-year U.S. Treasury notes as well as the most recently auctioned U.S. Treasury 30-year bond, plus the off-the-runs in the 20-year maturity range.

Investopedia explains One-Year Constant Maturity Treasury - 1-Year CMT
The U.S. Treasury publishes the one-year CMT value on a daily basis. Official weekly, monthly and annual one-year CMT values are published respectively. The monthly one-year CMT value forms a popular mortgage index to which many fixed period or hybrid adjustable-rate mortgages (ARMs) are tied.

Some mortgages such as payment option ARMs offer the borrower a choice of indexes. This choice should be made with some analysis. Different indexes have relative values which historically are quite constant within a certain range. For example, the one-year CMT index is typically lower than the one-month LIBOR index by about 0.1% to 0.5%. When considering which index is most economical, don’t forget about the margin. The lower an index relative to another index, the higher the margin is likely to be.