One which refers to a single entity (particle) of a system.
Source: PAC, 1986, 58, 437. (Interphases in systems of conducting phases (Recommendations 1985)) on page 441 [Terms] [Paper]
Cite as: IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.