The Ghiljī (Pashto: غلجي, pronounced [ɣəlˈd͡ʒi]; Persian: خیلجی, romanized: Xelji) also spelled Khilji, Khalji, or Ghilzai and Ghilzay (غلزی), are one of the largest Pashtun tribes. Their traditional homeland is Ghazni and Qalati Ghilji in Afghanistan but they have also settled in other regions throughout the Afghanistan-Pakistan Pashtun belt. The modern nomadic Kochi people are predominantly made up of Ghilji tribes. The Ghilji make up around 20–25% of Afghanistan's total population.

They mostly speak the central dialect of Pashto with transitional features between the southern and northern varieties of Pashto.^{[citation needed]}

According to historian C.E. Bosworth, the tribal name "Ghilji" is derived from the name of the Khalaj (خلج) tribe. According to historian V. Minorsky, the ancient Turkic form of the name was Qalaj (or Qalach), but the Turkic /q/ changed to /kh/ in Arabic sources (Qalaj > Khalaj). Minorsky added: "Qalaj could have a parallel form *Ghalaj." The word finally yielded Ghəljī and Ghəlzay in Pashto.

According to a popular folk etymology, the name Ghəljī or Ghəlzay is derived from Gharzay (غرزی; ghar means "mountain" while -zay means "descendant of"), a Pashto name meaning "born of mountain" or "hill people."

One theory of origin states that Ghiljis are likely to be descended from the Khalaj people. According to historian C.E. Bosworth, it seems very likely that the Khalaj people of the Gazna formed the core of the Ghilji tribe, who are usually referred to as Turks. The Khalaj were sometimes mentioned alongside Pashtun tribes in the armies of several local dynasties, including the Ghaznavids (977–1186).

According to The Cambridge History of Iran volume 3, Issue 1, the Ghilji tribe of Afghanistan are the descendants of Hephthalites.

The 17th-century Mughal courtier Nimat Allah al-Harawi, in his book Tārīkh-i Khān Jahānī wa Makhzan-i Afghānī, wrote a mythical genealogy according to which the Ghilji descended from Shah Hussain Ghori and his first wife Bībī Matō, who was a daughter of Pashtun Sufi saint Bēṭ Nīkə (progenitor of the Bettani tribal confederacy), son of Qais Abdur Rashid (progenitor of all Pashtuns). Shah Hussain Ghori was described in the book as a patriarch from Ghor who was related to the Shansabani family, which later founded the Ghurid dynasty. He fled Ghor when al-Hajjaj ibn Yusuf (Umayyad governor of Iraq, 694–714) dispatched an army to attack Ghor and entered into the service of Bēṭ Nīkə, who made him an adopted son. The book further stated that Shah Hussain Ghori fell in love with the saint's daughter Bībī Matō, fathering a son with her out of wedlock. The child was named by the saint as ghal-zōy‌ (غل‌زوی), Pashto for "thief's son," from whom the Ghilzai derived their name. The 1595 Mughal account Ain-i-Akbari, written by Abu'l-Fazl ibn Mubarak, also gave a similar account about Ghiljis' origin. However, it named the patriarch from Ghor as "Mast Ali Ghori" (which, according to Nimat Allah al-Harawi, was the pseudonym of Shah Hussain Ghori), and asserted that the Pashtuns called him "Mati". After the illicit intercourse with one of the daughters of Bēṭ Nīkə, "when the results of this clandestine intimacy were about to become manifest, he preserved her reputation by marriage. Three sons were born to him, vis., Ghilzai (progenitor of the Ghilji tribe), Lōdī (progenitor of the Lodi tribe), and Sarwānī (progenitor of the Sarwani tribe)."

Medieval Muslim scholars, including 9th-10th century geographers Ibn Khordadbeh and Istakhri, narrated that the Khalaj were one of the earliest tribes to have crossed the Amu Darya from Central Asia and settled in parts of present-day Afghanistan, especially in the Ghazni, Qalati Ghilji (also known as Qalati Khalji), and Zabulistan regions. Mid-10th-century book Hudud al-'Alam described the Khalaj as sheep-grazing nomads in Ghazni and the surrounding districts, who had a habit of wandering through seasonal pastures.