CC Madhya 20.329

yugāvatāra ebe śuna, sanātana
satya-tretā-dvāpara-kali-yugera gaṇana
Word for word: 
yuga-avatāra — incarnation of millenniums; ebe — now; śuna — hear; sanātana — O Sanātana Gosvāmī; satya-tretā-dvāpara-kali-yugera — of Satya-yuga, Tretā-yuga, Dvāpara-yuga and Kali-yuga; gaṇana — the chronological order.
Translation: 
“O Sanātana, now hear from Me about the yuga-avatāras, the incarnations for the millenniums. First of all, there are four yugas — Satya-yuga, Tretā-yuga, Dvāpara-yuga and Kali-yuga.