Thread:User talk:CodeCat/Autocannons are not machine guns/reply (16)

Of course, but when you say that sentence, you're using the word "car" and intend it to mean something specific. That's its definition; it's what people mean when they say it.

In this case, when people say "cannon", they sometimes mean something that could also be described by the word "autocannon". So in that sense they are synonyms. Therefore, one of the definitions of "cannon" is "autocannon".