In experimental particle physics at e.g. the LHC, the analysis of a certain process is often divided into categories described as "0-jet", "1-jet" and "2-jets", etc. Although it seems obvious, I want to have a deeper and more detailed explanation about the exact meaning of "1-jet" and how do we classify events as such.
In general, one will choose a rapidity range in the central region, $|\eta|<2.5$ e.g., and only jets within it will be counted. Then a minimum $p_T$, let's say 25 GeV for illustration, will be required too. So a 1-jet event will be an event with only 1 jet meeting those criteria; a 2-jet event, one with only 2 jets meeting them; and a 0-jet event, one with none meeting them. Those cuts depend on the particle process and observable one is interested in. I should add that there are other possible filters, two examples of which being (anti) $b$-tagging and central jet veto. The former requires a $b$ quark in a jet, or no $b$ quark for anti $b$-tagging. The latter is a variant to select 2-jet events: 2 hard jets well separated in rapidity, furthermore requiring there is not a 3rd jet with a $p_T$ above some threshold in the rapidity interval between the 2 hard jets.