Memory foam is a viscoelastic polyurethane foam that responds to body heat and pressure, softening where it is warmed and slowly moulding to the shape pressing into it before returning to its original form once the pressure is removed. This gives it a distinctive slow, cushioned feel compared with the quicker rebound of cold foam.
On a sofa, memory foam is often used as a top layer over a firmer support foam, combining pressure relieving comfort with the structural support the firmer layer provides. Used entirely on its own it can feel too soft and warm for some people, and it tends to respond more slowly in cooler rooms. See our buying guide for how cushion fillings affect overall comfort.