This refers to a process whereby gaseous molecules are ionized when they interact with other internally excited gaseous molecules or molecular moieties. Chemi-ionization and chemical ionization are two terms which should not be used interchangeably.
See also: associative ionization (in mass spectrometry), chemical ionization (in mass spectrometry)
Sources:
Orange Book, 2nd ed., p. 204 (https://media.iupac.org/publications/analytical_compendium/)
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 1547 (https://doi.org/10.1351/pac199163101541)