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.
To move in a vertical direction away from basal seat
To move in a vertical dimension towards basal seat
To movements on its tissue foundation especially to lateral forces
Against any kind of movement more the 24 µm in any direction