Monism
Monism is an old Greek concept that reality is One. What is that One is highly debatable. For e.g Some said there is only 1 substance and everything is derived from it like fire or air or water. Some said there only 1 personality and everything else is from it, then some said 1 is mind others said matter etc etc Point is that the reality becomes something unique and everything things else is either derived from it or it’s an arbitrary division or an illusion (depends which monism you believe in) Islam has nothing to do with monism. Tauheed is very different concept. Allah is one but that oneness is only related to Him being God and Creator. What we see around us and inside us (ruh/soul/consciousness) exist and will perish one day. Islam reached there’s 1 Khaliq or creator and everything else is Makhlooq or created.