It's simply when the shape of a protein is changed for some reason.
- What can cause a protein to be denatured?
- Heat, change in pH of the protein's environment, heavy metals with strong + or - charges can change the shape of a protein. Physical force can change the shape of some proteins, too.
Eggs have loads of protein, cook them up and the heat causes the proteins to change shape and the egg becomes more solid.
Cream has a lot of protein, whip it up and it and the physical force changes the shape of the proteins and it becomes more solid.
Milk has protein, add lemon juice ( acidic, change in pH ) and it separates into solid and liquid portions because the proteins have changed shape.
In each case the proteins are denatured.