deductionEnglishOxford American Thesaurusdeduction noun 1 the deduction of tax: subtraction, taking away, taking off, withdrawal, abstraction, removal, debit, docking, discounting; informal knocking off. ▷antonyms addition. 2 NI contributions are worked out on gross pay, before all deductions: stoppage, subtraction. 3 she had been right in her deduction that he was in love with someone else: conclusion, inference, supposition, hypothesis, thesis, assumption, presumption, suspicion, conviction, belief; reasoning; results, findings.