a (usually strict) subset of semantic garbage

Syntactic garbage is a (usually strict) subset of semantic garbage as semantic garbage is entirely possible for an object to hold a reference to another object without the latter object being used.

data that cannot be reached

Casually stated, syntactic garbage is data that cannot be reached, while semantic garbage is data that will not be reached.

data that is unreachable due to the reference graph

More precisely, syntactic garbage is data that is unreachable due to the reference graph