Among the province's known old names are Agrilion and Belokome, and how they relate to the modern one is argued over. In his book on Bithynia, Prof. Bilge Umar reads the original form of Agrilion as Akr(a)-ilion, meaning the throat of the peaks — the gap between summits.
The later name is better attested. The settlement that was the ancestor of Bilecik was called Belo Kome in the last Byzantine period, and there is no hesitation about where that village stood, since it is named in the work of the historian Pachymeres. But Belo has no meaning in that language. Ramsay held that Belo Kome was the Greek adaptation of the Turkish name Bilecik.
Umar argues the reverse. Apart from the final -cik, he writes, Bilecik has nothing to do with Turkish and no meaning in it; some reference works, among them the state-published Türk Ansiklopedisi, give the name as Bele Kome, read Vilo Komi, and it is natural for Bele — Vile — to have become Vile-cik and then Bilecik in Turkish mouths. He traces the name to the Bithynian language, of Thracian origin, or to Phrygian, also Thracian in origin, and notes that a castle founded in the Byzantine province of Thrace under Justinian, between 527 and 565, was called Beledina.
