Entry 1 sense Webster, 1913 oncotomy /(ŏṉ"kŏt*ō̇*my̆)/ · on·cot·o·my · IPA 01 n. The opening of an abscess, or the removal of a tumor, with a cutting instrument. 1. The opening of an abscess, or the removal of a tumor, with a cutting instrument.(Surg.)