A semisynthetic or partial chemical synthesis uses compounds isolated from natural sources (e.g. plant material or bacterial or cell cultures) as starting materials. These natural compounds are usually large and complex molecules. This is opposed to a classical chemical synthesis where large molecules are synthesized from a stepwise combination of small and cheap (petrochemical) building blocks.
Semisyntheses are usually used when the precursor molecules are too complex to be synthesized economically by a total synthesis.
One example is the synthesis of LSD from ergotamine, which is isolated from ergot fungus cultures.