horse parsley hor·se·pars·ley uncommon noun 1. European herb somewhat resembling celery widely naturalized in Britain coastal regions and often cultivated as a potherb SynonymsAlexanderAlexandersblack lovagehorse parsleySmyrnium olusatrum Related words Broader terms herb Parts Smyrnium WordNet 2.1 (Princeton University). WordNet 2.1 Copyright Princeton University. All rights reserved.