to break down (organic matter) or (of organic matter) to be broken down physically and chemically by bacterial or fungal action; rot
2.
chem to break down or cause to break down into simpler chemical compounds
3.
to break up or separate into constituent parts
4.
(tr) maths to express in terms of a number of independent simpler components, as a set as a canonical union of disjoint subsets, or a vector into orthogonal components