What the same houses sell for, on FHFA’s repeat-sales index. Measuring the same property twice is what separates a price move from a change in the mix of what happened to sell that month, which is the flaw in a median. It is quoted as an index rather than a price: January 1991 is set to 100, so a single reading means nothing on its own and the distance between two dates is the whole number.
FHFA via FRED · the purchase-only house price index, all US, monthly