The US National Postdoctoral Association defines a postdoctoral researcher as follows:
A postdoc is generally a short-term research position that provides further training in a particular field, and for individuals planning research careers in academia, government, or industry, the postdoc years can be an opportunity to develop independence, hone technical skills, and focus research interests.
When senior researchers apply for research grants, they will usually build in funded doctoral and postdoctoral roles as part of the project. This can result in job adverts for short-term PDRA/F positions.