This term is often, and wrongly, considered interchangeable with "coach bolt", which is UK/Australian slang for a lag bolt. This may seem a trivial inconsistency to the layman, but is a mixup which has cost innumerable companies lots of money and confused a huge number of people.
One may perform a quick test to determine whether an item is a carriage bolt or lag bolt/lag screw: Is it driven (tightened) by its head or can only nuts be driven up it? If the former, it's a carriage bolt, if the latter, it's <i>probably</i> a lag bolt.
Origin: Old French <i>cariage</i>, from <i>carier</i> ‘carry’; bolt (unchanged), Old English. There is no reliable information as to whether the carriage bolt got its descriptor through the manufacture of carriages/coaches or whether 'carriage' refers to the fact that such a bolt is a load-bearing item.