The
Concise Oxford English Dictionary defines a monkey as "a small to medium-sized primate typically having a long tail and living in tropical countries." While perfectly good for everyday speech, that's not a viable scientific definition, for two reasons. For a start, you can't just say "typically" - you have to define exactly what you're talking about.
The other problem, though, is more subtle, and relates to how we define groups of animals. As I've mentioned many times before, a natural group of animals is one that includes a single common ancestor and
all of its descendants. Apes are descended from monkeys, so, scientifically speaking, either apes are a special kind of monkey, or there is no such thing as a "monkey". You can take your pick, but scientists more commonly use the former option. A gorilla, therefore, is a kind of monkey, and it manifestly doesn't fit the definition given above.