What is Advisory Circular?

Legal Definition
Advisory circular (AC) refers to a type of publication offered by the Federal Aviation Administration (FAA) to provide guidance for compliance with airworthiness regulations. They define acceptable means, but not the only means, of accomplishing or showing compliance with airworthiness regulations. Generally informative in nature, Advisory Circulars are neither binding nor regulatory; yet some have the effect of de facto standards or regulations.

Advisory circulars typically refer to industry standards from SAE and RTCA. Some Advisory circulars are only a few pages long and do little more than reference a recommended standard; for example, AC 20-115B referencing DO-178B. Others, like AC 20-115C, are considerably longer; in this case giving guidance in how to transition from DO-178 revision B to C.
-- Wikipedia
Legal Definition
A publication sent out to help with code compliance. It gives tips and the regulations themselves.