There are a number of different terms for a male horse, depending on its age and whether it has been castrated:
For comparison, see also female horse.
The age definitions are interpreted differently depending on who is using them. For example, a colt may be defined as under 4 years of age, but some horse organisations define a colt as being under 3 years of age while others use 5 years.
When calculating age, racing organisations tend to use a specific date (e.g. 1st January) regardless of the actual birthday. For example, a horse born on 1st May 2009 would be considered a year old on 1st January 2010 (rather than on 1st May 2010). The reason for this is that some races are for specific ages (e.g. 3-year olds only), while other races which accept multiple ages will handicap horses differently depending on their age. To have to track each horses actual birthday to determine if it is eligible to race and to calculate its handicap would be complex, so by using a common birthday for all horses one only needs to use the birth year, which is much simplier.