Here's why:
* "Definition" refers to the explanation of the meaning of a word or phrase.
* "Words" and "meanings" are both elements that a definition explains.
Therefore, it makes sense to say "definition of words or meanings" to encompass both aspects.
You can also say:
* "Definitions of words"
* "Definitions of meanings"
* "Definitions of terms" (if you're referring to technical or specialized language)
The choice depends on the specific context and what you want to emphasize.