In mathematics, odds are the chance that an event will happen compared to the chance that it will not happen. For example, if there is an equal chance that it will rain or that it will not rain, the odds of it raining are 1:1 (which is read as "1 to 1"). If it is twice as likely that it will not rain, then the odds of it raining are 1:2 (read as "1 to 2").