what is the meaning of "core body of research" in this sentence?
The concept of critical thinking we adhere to reflects a concept embedded not only in a core body of research over the last 30 to 50 years but also derived from roots in ancient Greek.
A core body of research refers to a set content area. That is, the school or organization in question utilizes a particular concept that was developed from a certain specific collection of data (which was researched and collected over a period of 50 years).
A more simplistic example would be if I wrote a "How To Survive on a Desert Island" reference book based on information I gleaned from watching every episode of Gilligan's Island dozens of times. My core body of research in this case was a TV show.