.ac (short for academic) is a second level domain for academic establishments, such as universities, colleges and research institutes in a number of countries such as New Zealand (.ac.nz), the United Kingdom (.ac.uk), India (.ac.in), Japan (.ac.jp), Austria (.ac.at), Belgium (.ac.be), Iran (.ac.ir), South Africa (.ac.za), and many more. Many countries use .edu for the same purpose, for example .edu.au in Australia, .edu.my in Malaysia. Still other countries (examples include France and Switzerland), do not maintain a second level domain specifically for academic institutions. Rather, each institution will have its own second-level domain (such as sorbonne.fr for the Sorbonne, or tum.de for the Technical University of Munich).

Though similar, there is a slight distinction between .edu and .ac, as .edu is generally restricted to educational institutions, whereas .ac can generally be used by any academic organization, including museums and research institutions. For nations in which .edu is used, sites for these non-educational academic organizations would reside in the .org domain instead.