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Centrifugation separates suspended material by its motion relative to a rotating liquid. A centrifuge rotor creates an outward centrifugal acceleration in the rotating reference frame; material denser than its surroundings tends to move outward, opposed by drag force. Collected material can form a centrifugation pellet, leaving a supernatant, or remain in separated zones within a density gradient. The useful question is which physical difference the separation exploits: a difference in sedimentation coefficient or a difference in buoyant density.

Centrifugation makes otherwise slow settling useful for studying biological cells, organelles, proteins and DNA. Separation and identification are different steps: a centrifugation pellet is a collected fraction, not proof that it contains only one kind of organelle. The lysozyme wall-removal experiment illustrates a complementary readout: testing material released into the separated liquid can be more informative than inspecting the retained solids.

Differential centrifugation successively separates centrifugation pellets from supernatants by differences in settling rate. A first centrifugation pellet can enrich faster-settling material; a later centrifugation pellet from its supernatant can enrich slower-settling material. The fractions overlap because particles have different starting positions and can become trapped. Differential centrifugation is consequently an enrichment method, not automatic identification or complete purification.^[ref]

A supernatant is the liquid remaining above settled or pelleted material. It can contain dissolved molecules and particles that have not reached the centrifugation pellet; a visually clear supernatant need not be chemically empty.