What is a vaccine-derived poliovirus?

A vaccine-derived poliovirus (VDPV) is a strain of the weakened poliovirus that was initially included in oral polio vaccine (OPV) and that has changed over time and behaves more like the wild or naturally occurring virus.

https://www.cdc.gov/vaccines/vpd/polio/hcp/vaccine-derived-p...

Notably, this isn't someone reverse engineering polio from vaccines. (it's still endemic so no need)

Although arguably that's exactly what the virus did itself.