The spread between 10-year and 2-year US Treasury yields. The most widely watched yield curve indicator in the world.
Every US recession since 1955 has been preceded by an inversion of this spread. The Fed, banks, and institutional investors watch it daily as a recession early warning.
Positive = normal curve, growth expected. Near zero = flattening, watch closely. Negative (inverted) = historically a recession signal within 12-18 months.