I think that you give is an excellent definition of what the word "evolution means" ... it means "change in response to environment."
The only thing I would add is that it is specifically change *at the population level* ... individuals also change in response to environment (a cat living in the wild will have a very different body than the same cat living in a domestic environment) ... but that is not "evolution".
But other than that, what you have described is an excellent definition of "evolution." It's just "change."
Darwin's theory of natural selection goes one step further to describe HOW that change happens in nature. But even there, creationists have a hard time disputing this, and will reluctantly admit that that does occur ... although they will insist that this is "adaptation, not evolution." But that is just mincing words. Regardless of what you call it ... that process of slow change in response to environment is undeniable.
Where creationists *disagree* is on whether this same process (change in response to environment) can cause new species (what evolution people call "speciation" and what creationists call "macroevolution"). *That* is where your creationist friend will draw the line.
The answer that creationists always seem unable to "get":
Three words: branching, branching, branching.
A new species is always the result of one species branching into two. How does this happen? When two subpopulations of a species get genetically isolated ... a migration, a river bisecting a valley, a continent splitting apart, etc.
It is after two populations become isolated, that the same old process of slow change in response to environment can produce two slightly different organisms that cannot interbreed. They are then forever freed to change in completely different ways ... and two slightly different organisms become two *very* different organisms.