Arbitrarily defined part of the universe, regardless of form or size, e.g. for clinical chemistry, patient, patient plasma, patient urine.
Source:
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 995 (https://doi.org/10.1351/pac199668040957)