I don’t recall hearing of the words “reify” or “reification” in my OOP years, but that may be because I studied aerospace engineering in college, not computer science. Since learning FP I often see those words, so I thought I’d try to understand their meaning.
The short answer is that the main definition seems to be:
“Taking an abstract concept and making it concrete.”
For the longer answer, I found the following definitions and examples of reification.