Numeral or letter or a combination of both that identifies position(s) in a structure.
Note: A locant appears in a name or general expression such as ‘2-substituted’.
Source:
PAC, 2021, 93, 963. 'Structure-based nomenclature for irregular linear, star, comb, and brush polymers (IUPAC Recommendations 2020)' on page 966 (https://doi.org/10.1515/pac-2020-0103)