the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
A hadîth is described as gharîb (strange) if it comes to us by way of one narrator only.
If the hadîth is only known in any way by the narration of that single narrator, then it will be called “gharîb mutlaq” or strange in an absolute sense
But, if it is narrated in another way, other than the one mentioned by that single narrator, then it will be called gharîb nisbî, strange by this or that way only.
A gharîb hadîth is also refrred to as fard
Ibn Hajar writes:
Gharîb and fard are the same linguistically as well as terminologically. The only difference is that the scholars of hadîth terminology tend to use one term more often for one purpose than the other. The term fard is most often used to refer to a hadîth that is fard mutlaq, while the term gharîb is more often used to refer to a hadîth that is fard nisbî.
This distinction is only with respect to the terms as names. With respect to their verbs from which those names derive, they are used without any distinction being made between them. They say that somebody narrated it by himself with both the verbs “tafarrada bihi” and “agraba fîhi”.
Another term sometimes used with the same meaning is fâ’idah. This terminology has been used by Abû `Awânah al-Harrânî about the narrations of Zuhayr b. Muhammad al-Khurasânî and by Ibn `Adiyy about Zam`ah b. Sâlih.
Ahmad b. Hanbal said: “If you hear hadîth scholars say that a particular hadîth is gharîb or fâ’idah, then know that it is a mistake.”
And Allah knows best.
Source: Islam Today