Constant Maturity
  
Something we Shmoop writers will never ever have. Not even a glimmer.
Okay, so a package of bonds (think: short-ish term U.S. Government Treasury paper) comes to maturity all the time. Like...in 10,000 bond offerings, something is maturing more or less every day. The constant maturity rate takes an average imputed interest rate for that maturing process and then quotes it as the rate of that package.
Think about it like the yield on commercial paper in a money market fund. It's just what that packaged bond offering yields, with the varying dates of maturity factored in.