Reductionism is a philosophical idea that suggests that complex phenomena can be explained in terms of simpler or more fundamental phenomena. It is a view that asserts that entities of a given kind are identical to, or are collections or combinations of, entities of another (often simpler or more basic) kind or that expressions denoting such entities are definable in terms of expressions denoting other entities. There are several types of reductionism, including:

• Ontological reductionism: the belief that everything in the world can be reduced to a single fundamental reality.
• Methodological reductionism: the scientific attempt to provide an explanation in terms of ever-smaller entities.
• Theory reductionism: the suggestion that a newer theory does not replace or absorb an older one, but reduces it to more basic terms.

Reductionism can be applied to any phenomenon, including objects, problems, explanations, theories, and meanings. Reductionism is often associated with the idea that a system is nothing but the sum of its parts.

Reductionism is not a philosophy, but rather a hypothesis about the world. Reductionism is important for scientific understanding, as it allows scientists to break down complex phenomena into simpler parts that can be studied and understood. However, reductionism has been criticized for being overly conservative and not making room for a plurality of sciences and resultant methodologies.

In the context of biology, reductionism means attempting to explain all biological phenomena in terms of their underlying biochemical and molecular processes. Reductionism in biology encompasses a set of ontological, epistemological, and methodological claims about the relations between different scientific domains. Reductionism in biology has often been framed in terms of "reductionism" vs. "anti-reductionism".