“Mind over matter” has been used in several contexts, such as mind-centric spiritual doctrines, parapsychology, and philosophy. I’d say the simplest definition is “will power.”
The Merriam-Webster Dictionary defines mind as “the element or complex of elements in an individual that feels, perceives, thinks, wills, and especially reasons” and mind over matter as “a situation in which someone is able to control a physical condition, problem, etc., by using the mind.”
“Mind over matter” has come to mean overcoming adversity or meeting a challenge by exercising one’s willpower.
The phrase first appeared in 1863 in The Geological Evidence of the Antiquity of Man by Sir Charles Lyell (1797–1875) and was first used to refer to the increasing status and evolutionary growth of the minds of animals and man throughout Earth history.
Another related saying, “the mind drives the mass,” or “mind moves matter” was coined almost two millennia earlier in 19 BCE by the poet Virgil in his work Aeneid.
In the field of parapsychology, the phrase has been used to describe paranormal phenomena such as psychokinesis. It also relates to the ability to control the perception of pain that one may be experiencing.
Here’s another view from The Magician King (2011) by Lev Grossman: “Magic: it was what happened when the mind met the world, and the mind won for a change.”
It’s fun to think the mind has power over the physical world. But that may simply be the power of the mind to change one’s own attitude.