A probability is a number between 0 and 1 that measures how likely an event is: 0 for an impossible event, 1 for a certain event. For equally likely outcomes, it equals the number of favorable cases divided by the number of possible cases. The probability of the complementary event is 1 minus that of the event.
Example
Rolling a 6 with a fair die: P = 1/6, or about 0.17.