Theres not really a definition or cause for it. When babies cry alot, for hours straight in a day, they call it colic. So if your lucky enough to figure out whats causing it, you can make your baby happy and "cure" it.
"Colic" is a medical term for "we don't know why the baby is crying but we must have a medical term." It's defined by bouts of unexplained crying.
Docs honestly don't know what causes the crying jags. If it's happening in the evening, it may well be from a full day of stimulation. Baby cries to make the world go away. The "witching hour" is a common phenomenon that is cured by time. But you can help things along by slowing down the stimulation as the sun goes down.
A lot of times its tummy trouble. Its something they are born with, they can't "catch" it. A lot of moms with colic switch to soy formula and say it helps, but yeah...its not something you can catch lol