I apply the apparatus of David Hitchcock’s book Definition (2021) to the task of defining antisemitism. Hitchcock distinguishes three basic acts of defining: stipulating a meaning, reporting a meaning, and advocating a meaning. An initial stipulation introduced the word ‘Semitismus’ into the German language as a synonym for ‘Judenthum’ (‘Jewishness’). The choice of term risked impact equivocation, and the stipulator’s use of either name assumed falsely that all Jews share the described characteristics. To illustrate the task of reporting a meaning, I use a sample of 10 sentences using the term ‘antisemitism’ to evaluate 11 proposed reportive definitions. I rephrase the resulting tentative definition in the Natural Semantic Language developed by Goddard and Wierzbicka. Two documents advocate a meaning for ‘antisemitism’, in the sense that they take a position on what kinds of speech or action count as antisemitic. The Jerusalem Declaration on Antisemitism, issued by a group of researchers in 2021, is superior in many respects to the resolution on antisemitism adopted in May 2016 by the International Holocaust Remembrance Alliance (IHRA). Hence individuals and organizations concerned with monitoring antisemitism should use the Jerusalem Declaration as a guide rather than the IHRA resolution. Consideration of the declaration leads to a revised reportive definition: Antisemitism is discrimination, prejudice, hostility, or violence that targets people or institutions regarded as ethnically Jewish, just because they are Jewish.