Nobody knows what it really is, but everyone has an opinion.
In the typical situation, the baby starts to have crying periods about two to three weeks after birth. When the baby cries, he is often inconsolable, though if he is walked, rocked or taken for a drive, he may settle temporarily. For a baby to be called colicky, it is necessary that he be gaining weight well and be otherwise healthy. The notion of colic has been extended to include almost any fussiness or crying in the baby, and this may be valid, since we do not really know what colic is. There is no treatment for colic, though many medications and behaviour strategies have been tried, without any proved benefit.