In 2009, Epic Games released what was known as the Unreal Development Kit which was a free version of Unreal Engine 3. Following that up Epic later released Unreal Engine 4, first as a subscription model, but later available for free, including the entire source code. Later again, a major upgrade including Lumen and Nanite features allowing

Source: Hot Hardware – Unreal Engine 5.1 Releases With Epic Graphics Upgrades And Robust Next-Gen Game Support