The spread between long and short government bond yields
Long yields minus short yields. When short rates exceed long ones the curve is inverted, which has preceded most recessions.
Government bonds of different maturities normally yield more the longer they run, because lenders want compensation for tying money up. When markets expect the central bank to cut rates because the economy is weakening, long yields fall below short ones and the curve inverts.
In the United States an inversion of the ten-year minus three-month spread, sustained for a quarter, has preceded every recession since 1968 with a lead of six to eighteen months. The curve usually re-steepens before the recession arrives, which is why worldstat.us moves the indicator to Post-trigger rather than Normal when it un-inverts. Other regions use the closest available short rate as the short end.