Def. 4. But parts when it does not measure it.
Def. 5. The greater number is a multiple of the less when it is measured by the less.
These definitions assume that "measures" is understood. It is clear what is meant by these definitions, namely, m measures n if m divides n, that is, there is some number k (greater than 1) so that mk = n. But division and multiplication haven't been defined yet, so this definition is inadequate, too. The problem, of course, is that Euclid has no foundations for number theory. The concepts of arithmetic were as obvious to him as as they were all others after him until the skeptic formalists of the late nineteenth century recognized a need for foundations of number theory.
Book VII Introduction - Definitions 1 and 2 - Definitions 6 through 10.