unreachable from the program's root set

Garbage is generally classified into two types: semantic garbage that is any object or data never accessed by a running program for any combination of program inputs, and syntactic garbage that refers to objects or data within a program's memory space but unreachable from the program's root set.

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 is unreachable due to the reference graph

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

that cannot be reached

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