غُلَامٌ: a servant, indefinite and unspecified.
غُلَامُ رَجُلٍ: a man’s servant, still indefinite but more restricted.
غُلَامُ زَيْدٍ: Zayd’s servant, definite through a known name.
The lexical meaning of ghulām stays constant, while the listener’s ability to identify its referent changes. This distinguishes taʿrīf from takhṣīṣ.