An ion formed by the combination of more ions or atoms or molecules of a chemical species often in association with a second species. For example,
[(H2O)nH]+ is a cluster ion.
Source:
PAC, 1991, 63, 1541. 'Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)' on page 1549 (https://doi.org/10.1351/pac199163101541)