= Defined medium
A <defined medium> is a <culture medium> whose individual chemical constituents and specified amounts are known. This makes <chemical composition> an explicit experimental variable. It differs from a <complex medium>, which contains ingredients with an incompletely specified molecular composition. A <defined medium> can help investigate which <nutrients> an <organism> requires under stated conditions.<Bacteriology Culture Guide by ATCC>[Growth Media, Bacterial Culture Media: defined media and nutrient-requirement studies; Glossary, Defined medium]{ref}
Known composition permits a quantitative inventory. Consider an illustrative <chemical solution> containing <glucose> at $10\,\mathrm{mmol\,L^{-1}}$ in a <volume> of $0.250\,\mathrm L$. Its <amount of substance> is
$$
n=cV=(10\,\mathrm{mmol\,L^{-1}})(0.250\,\mathrm L)=2.5\,\mathrm{mmol}.
$$
If <dilution> increases the final <volume> to $1.000\,\mathrm L$ without consuming <glucose>, its final <molar concentration> is $2.5\,\mathrm{mmol\,L^{-1}}$. Because each <glucose> molecule contains six <carbon> atoms, that is $15\,\mathrm{mmol\,L^{-1}}$ of carbon atoms supplied by this ingredient. Molecular concentration and elemental inventory answer related but different questions.
This calculation is an accounting illustration, rather than a complete <culture medium> formulation. It predicts neither how much <biomass> will form nor whether other required <nutrients> are present. A known starting inventory makes such questions testable; it does not supply their biological answers.
Composition and physical presentation are separate classifications. A liquid preparation and an <agar>-solidified preparation can be compared by their physical state, while chemically defined and complex describe what is specified about their ingredients.<Bacteriology Culture Guide by ATCC>[Growth Media, Bacterial Culture Media: defined/complex and liquid/solid distinctions]{ref}
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