http://www.jaa.nl/jar/jar.html
Select JAR-25, then applicability. For some reason I can't get it to load at the moment.
Having said which, nobody will ever apply part 25 if they can't get away with 22, 23 or VLA. So, in reality, it's anything that doesn't meet one of the other codes.
On that basis, a more realistic definition is found by looking at JAR-23's definitions section. Anything that can't meet that will be a part 25 aeroplane.
G