This is GoDocs terminology
Index: The weekly average yield on United States Treasury securities adjusted to a constant maturity of five (5) years, as published in Federal Reserve Statistical Release H.15 (519). If the Index or the information to calculate the Index or Current Index becomes unavailable, Lender will choose a new Index and Current Index based upon comparable information. The selection of an alternate Index and Current Index shall be made in Lender's sole discretion. Lender will give Borrower notice of such selection.