A particle much smaller than the @W06659@ of the radiation in the medium. In practice, all dimensions of a particle considered small must be less than about one-twentieth of the @W06659@ employed.
Source: Purple Book, 1st ed., p. 65 [Terms] [Book]
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.