Firmness describes how hard or soft a mattress feels near the surface. Support is different: a softer mattress can still have a strong support core, and a firm mattress can still be a poor fit for a particular sleeper.
What changes perceived firmness?
- Body weight and shape
- Sleeping position
- Comfort-layer materials
- Thickness and compression
- Room temperature for some foams
Use labels as a starting point
Terms such as plush, medium and firm are not perfectly standardized across brands. When possible, compare trial policies and test the exact model rather than relying on the label alone.