Denture stability refers to resistance of a denture:
Stability (GPT-9): 

That quality of maintaining a constant character or position in the presence of forces that threaten to disturb it; the quality of being stable; to stand or endure.
The quality of a complete or removable partial denture to be firm, steady, or constant, to resist displacement by functional horizontal or rotational stresses.
Resistance to horizontal displacement of a prosthesis.