It's appropriate that we should still call this condition by its medieval name "colic," since we know about as much about its causes today as we knew 800 years ago.
A colicky baby is one who cries a lot and can't easily be pacified. Though there isn't a full agreement on the definition of colic, a common one is that if an otherwise healthy baby cries for more than 3 hours a day, more than 3 days a week for at least 3 weeks, it's colic. Colic affects boys and girls equally, and the incidence is also similar between breast and formula fed infants, as well as full term and preterm infants. It tends to run from the age of 3 weeks to the age of 3 to 4 months.