Colt Starting
I think of colt starting as any of the first work done with a horse, from gentling and halter breaking, to the first few rides. The way the horse is first handled is important because good habits and responses are easier to start at the beginning rather than change later.
If colt starting were compared to building a house, it would be the foundation, and any training beyond that is the kind of house you build on the foundation. Good foundations make everything else possible.