Ali Gaji was the Mai (ruler) of Bornu Empire from 1472 or 1476 until 1503 or 1507.^[1][2][3] He is regarded as one of the "greatest rulers" of the empire and is attributed with ushering in the second era of Kanem-Bornu, following a century-long civil war that had divided the realm. He implemented reforms and put an end to internal conflicts that had plagued the empire and waged several successful wars with his neighbours. Additionally, he founded Birnin N'gazargamu, a capital city that remained the seat of the empire for over three centuries. During his reign, Bornu regained its prestige and was noted for its participation in the trans-Saharan trade, as noted by the Arab traveler Leo Africanus.^[4][5]

Mai Ali is known by several different names and nicknames. In accordance with the Bornu tradition of tracing descent through the mother, he is commonly referred to as Ali Gaji Zenamami after his mother, Zainam.^[6]: 122 Additionally, he is also called Ali Gaji Dunamami after his father who was Mai Ahmad Dunama IV.^{[6]: 74–75}

The Gaji or Ghaji in his name means 'the little' or 'the younger'.^[4]: 267 The name is usually given to the last child born in a family.^[7] Historian C.C. Ifemesia has pointed out that sometimes the Gaji in his name is mistakenly confused with Ghazi.^[4]

The onset of the second era of the Kanem-Bornu empire is widely attributed to the reign of Mai Ali.^[5] The century leading up to his rule had been marred by a protracted civil conflict within the empire, pitting the descendants of Mai Idris, the 26th ruler of Kanem, against those of Mai Dawud, the 27th ruler, both being sons of Mai al-Haj Ibrahim Nikale, the 20th ruler of Kanem. This internal strife eventually led to the loss of Kanem to the Bulala in the late 14th-century. Ali was a descendant of Mai Idris, and it was his direct lineage that held sway throughout the 16th-century.^[4]: 267 During the civil war, Ali fought and killed Mai Uthman ibn Kadai and, afterwards, made Said (Umar) the next Mai.^{[4][6]: 114}

Mai Ali instigated vital governmental reforms, bringing an end to the century-long civil wars that had torn the empire apart. He engaged in several military campaigns, including battles against the Bulala in the Kanem civil war to the east, confrontations with Kwararafa in the south, and the extraction of tribute from a number of Hausa states in the west, including Kano, the commercial centre of the region. Furthermore, he expanded the empire northward into Borku and Tibesti.^[5] To centralize his administration, he erected the large capital of Birnin N'gazagamu, situated on the southern bank of the Komadugu Yobe River, near the border of what are now Nigeria and Niger.^[4][6] This capital remained the focal point of power for the empire's rulers for three and a half centuries, until it was destroyed during the Sokoto jihad in 1812.^[5]

Under Mai Ali's rule, Bornu also gained notable prestige. The Arab traveller Leo Africanus underscored the significance of Bornu in trans-Saharan trade during Ali's reign. This period marked Bornu's initial appearance on European maps of Africa produced by Portuguese cartographers in the late 15th-century.^[5]