Praise be to Allaah.
A ghareeb hadeeth is one in which there is some kind of uniqueness. There are many kinds of uniqueness, the most important of which are two:
1 - Absolute uniqueness, which is where a particular narrator - at any stage of the isnaad - is the only one who narrated this hadeeth, and no one else narrated it alongside him. So in this case the ghareeb hadeeth is one which is only known to be narrated from the Prophet with one isnaad.
An example of this is the hadeeth “Actions are but by intentions.” This hadeeth has only one proper isnaad. It was narrated by Yahya ibn Sa‘eed al-Ansaari, from Muhammad ibn Ibraaheem al-Taymi, from ‘Ilqimah ibn Waqqaas al-Laythi, from ‘Umar ibn al-Khattaab.
2 - Relative uniqueness, which means that one of the narrators of a particular hadeeth was the only one to narrate it from his shaykh, and no one else narrated it from his shaykh, even though the hadeeth was narrated via a number of isnaads, but none of his students narrated the hadeeth from him except one narrator.
An example of this is the hadeeth narrated by ‘Eesa ibn Moosa Ghunjaar, from Abu Hamzah al-Sukkari, from al-A‘mash, from Abu Ayyoob al-Sakhtiyaani, from Muhammad ibn Sireen, from Abu Hurayrah, who said: The Prophet (blessings and peace of Allah be upon him) said: “Do not call grapes al-karm.”
Al-Tabaraani said: No one narrated this hadeeth from al-A‘mash except Abu Hamzah al-Sukkari.
See how Abu Hamzah al-Sukkari was the only one who narrated this hadeeth from al-A‘mash, so his uniqueness is in relation to his shaykh al-A‘mash, but it is not absolute uniqueness, as the hadeeth was narrated via many isnaads from a number of the Sahaabah.
And Allah knows best.