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as a verb this is biochem “slang” for using a centrifuge to spin a sample really fast so that undissolved things separate from the liquid . As a noun, it refers to the solid part (consisting of the undissolved things) that has “pelleted out”. For example, when you purify proteins, you break open (lyse) the cells containing them – this gives you a “lysate” and then you want to separate the soluble things (including your protein, unless you’re trying to purify a membrane protein) from the insoluble “gunk” like the membrane pieces. So you centrifuge the lysate – this spins the lysate really really fast and the heavy & insoluble stuff gets pulled to the bottom of the tube or bottle, forming a pellet of solid stuff, above which is the liquid part, which we call the supernatant.