«وَهُوَ مَا يَنْفَرِدُ بِرِوَايَتِهِ شَخْصٌ وَاحِدٌ فِي أَيِّ مَوْضِعٍ وَقَعَ التَّفَرُّدُ بِهِ مِنَ السَّنَدِ»
A gharib report is one in which a single person is alone in transmitting it at some point of the isnad.
The key idea is tafarrud, a specific instance of solitary transmission. Here gharib does not mean that the subject matter is strange or objectionable; it refers to the distinctive pattern of transmission.
For example, one Successor might be the only person transmitting a particular report from a Companion, after which numerous later narrators spread it. The solitary early point remains. Similarly, several early narrators do not cancel a singular bottleneck arising later in a specific version of the report.