Substance: "The tangible matter of which a thing consists; a specific type of matter, esp. a homogeneous material with a definite composition." - Collins English Dictionary This class represents a substance, either manufactured or found naturally, which might be referenced by clinical records. This class may represent ingredients in a medication, a substance to which the patient was exposed or which the patient consumes, or an analyte, among many other potential uses. "The Substance entity is used to describe any material that functions as an ingredient of the Medicine." - HL7 Version 3. "The actual ingredient - either a substance (simple ingredient) or another medication." - HL7 FHIR, Medication.ingredient.item[x] "A homogeneous material with a definite composition." - HL7 FHIR, Substance