It's not my definition. I just happen to agree with a lot of others, many who don't post here. It's also not new. I just remind people of a view, you might never heard of, because you never were involved in Lisp meetings, Lisp conferences or Lisp standardization efforts. Lisp is old, and many younger people don't know much of its history and evolution.
See for example Pitman, Lambda the ultimate political party
http://www.nhplace.com/kent/PS/Lambda.html
Published in Lisp Pointers (a regular Lisp magazine, published in the 80s/90s) in 1994.
> Lisp users are accustomed to being able to write programs that take program texts in different dialects (or older versions of the same dialect) and process them to bring them up to date. No single common linguistic feature supports this. And yet, Lisp users are accustomed to expecting that there will be some way to accommodate the needs of compatibility and translation.
That's is what makes Lisp, in my opinion. Sharing of code, implementations, libraries, books, tools, ...
A language spectrum serves a diverse community. It is not a dead body of abstract and vague principles. It grows out of sharing and communication.
Kent knows the Lisp and Scheme landscape very well. He worked as developer, in standardisation efforts (both Lisp and Scheme), as an implementor, editor, author, etc.