Today I was watching a video of iDeactiveMC and found out that there is a old version of Minecraft called Minecraft Indev.
Please explain me exactly what Minecraft Indev is?
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Indev, literally meaning "in development", was the pre-beta version of Minecraft between December 2009 and February 2010. It was succeeded by Infdev, and then Alpha.
Basically, Minecraft development moved through a number of phases.
There's not a whole lot to explain beyond this and the linked wiki article :)