carriage bolt 

(noun, USA/SAE Standard) A bolt having a round head with a square, ribbed, or fluted boss underneath it. The function of the boss being to bite into timber to prevent the bolt turning while the nut is being tightened.

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> &#8216;carry&#8217;; 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.

Goddamn, I hate these translated instruction manuals. This thing just tried to tell me to use a screwdriver to tighten a carriage bolt!

carriage bolt by Ike January 26, 2005