An expansion of the SMILES language that describes a particular specific or generalized substructure.
Examples: The SMARTS that describes any aliphatic ester of a carboxylic acid is "C(=O)OC"; the SMARTS that describes any aromatic ester is "C(=O)Oc"; and two SMARTS that describes both are "C(=O)O[c,C]" and "C(=O)O[#6]".
Source:
PAC, 2016, 88, 239. 'Glossary of terms used in computational drug design, part II (IUPAC Recommendations 2015)' on page 256 (https://doi.org/10.1515/pac-2012-1204)