The belief that there is only one type of stuff in the universe. Either mind* (
idealism), or
mass/energy (
materialism).
The most common monism these days is materialism, as it seems that there is little room or use for the mind* in the biology of the brain.
If everything can be explained by traditional physics, why bother with non-physical stuff?
Idealism is interestering because (As far as I am aware) no-one can come up for a good argument against the idea that everything only exsists in our heads.
Dualistic interactionism (A type of dualism) is what most people believe in. Monism seems to be mostly used by people who are too scientific or philosopical to see the obvious. The obvious is overated anyway.
*Mind being different from thought. You might want to use the word spirit or soul in place of mind.
An Ethical Monist would be a person who has only one source of moral values, for example 'do the most good for the most people'. Having only one source of values keeps ethical systems simple.