The philosophical belief that everything is of one essence or energy. Those who believe this way would contend that seeing things as opposites that are contending or seeking balance with each other is due to our misunderstanding of reality or as a manifestation of reality. For example, in some Wiccan traditions there is an underlying deity who can only be understood and experienced through its first manifestations as a God and Goddess. The idea of the mind and body being two separate things would be a misunderstanding according to monists.
The opposite of monism is dualism.