This is used in a specific way on this website. It means how far in total the heavier parts of your body sink into a mattress in total. It determines spinal alignment rather than pressure relief.
This is used in a specific way on this website. It means how deeply you sink into a comfort layer material and the depth of the pressure relieving cradle that is formed. It is different from "sinking down".
The goal of perfect support is to keep the spine in its natural alignment in all sleeping positions.
A term often misused in the industry. It is a measure of how well a mattress supports and maintains the neutral alignment of the spine and joints. The primary support or deeper layers are mainly responsible for stopping the heavier pelvic girdle from sinking down too far into the mattress relative to the other lighter parts of the body while the upper or comfort layers provide the secondary or lighter support which fills in the gaps in the sleeping profile (like the waist or lower back) and helps to maintain the inward curves of the body and the spine in natural alignment.
This is the 65% IFD (how much weight it takes to compress a material 65% of it's thickness) divided by the 25% IFD. It is a measurement of the progressive resistance of a material. Higher quality materials will have a higher progressive resistance and although it is less well known or talked about in the industry, it is at least as important as ILD in determining the comfort and support qualities of a material.