Odds are a tool for expressing relative chances. If the odds of a tree in a forest being sick versus healthy are 2 : 3, this says that there are 2 sick trees for every 3 healthy trees. (The probability of a tree being sick, in this case, is 2/5 or 40%.)
Odds are expressed in the form "X to Y", e.g. "7 to 9 for X versus Y", more compactly written as .
The representation of chances as odds is often used in gambling and statistics.