The odds of
an event happening is a measure of the chance of that event happening
expressed using a ratio.

For example the odds of obtaining a three
with one roll of a die is 1 to 5 (1:5) as only 1 face has a three,
to 5 faces with other numbers. Compare this with the probability
of the same event which is 1/6.

The odds of drawing a jack, queen,
king or ace from a standard pack of cards is 4:9.