Denaturation midpoint of a protein is defined as the temperature (Tm) or concentration of denaturant (Cm) at which both the folded and unfolded states are equally populated at equilibrium (assuming two-state protein folding).
The midpoint of protein denaturation, often referred to as the melting point (Tm), or the concentration of a denaturing agent (Cm), is the temperature or concentration where both the folded and unfolded forms of the protein are equally likely. This assumes a two-state model for protein folding.