noun, formally a plural of, agendum but usually used as a singular with plural, agendas or agenda. 1. a list, plan, outline, or the like, of things to be done, matters to be acted or voted upon, etc.: The chairman says we have a lengthy agenda this afternoon. Origin of agenda. 1745-1755. 1745-55; < Latin, plural of agendum that which is to be done, gerund of agere to do; the plural orig.