By one measure, the yield curve inverted on Monday Dec 3: The interest rate on five-year Treasury bond slipped bleow the rate on three-year bonds.
That's a worrying sign because rates on longer-term bonds are typically higher than those on shorter-term bonds, and such inversions are associated with recessions.
So far, however, I would consider the yield curve only "partially" inverted.
A full inversion would be interest rates on two-year Treasury bonds rise higher than rates on 10-year bonds.
In the last 40 years, each time this has happened, the US economy has entered a recession soon afterwards.
This makes an inverted yield curve the most reliable indicator macroeconomists have for predicting a recession. The last two times the yield curve inverted, in 1998 and 2008, the debate among economists was whether this time would be different. In both cases, it wasn't.