the IRSIRS, the /ˌaɪ ɑːr ˈes/ (the Internal Revenue Service) the government organization in the US which is responsible for collecting national taxes, especially income tax (based on how much money you earn) and corporation tax (based on how much profit a company makes). People in the US have to send an income tax return by April every year (=an official form showing how much tax they have paid and how much they owe).