.ac is the Internet country code top-level domain ( ccTLD) for Saint Helena, Ascension and Tristan da Cunha. It is administered by NIC.AC, a subsidiary of the Internet Computer Bureau based in the United Kingdom.
Registration for this domain is open to anyone. The registry accepts registrations of internationalized domain names.
.ac is marketed by some domain-brokers as a domain for the city of Aachen in Germany, using the analogy to the automotive license plate designation ("AC") for the city.
The sequence .ac (short for academia) is in use in many countries as a second-level domain for academic institutions such as universities, colleges, and research institutes. In the United Kingdom and Japan, for example, academic institutions use domain names ending in .ac.uk and .ac.jp respectively.
Many countries use .edu for the same purpose, such as Australia (.edu .au) and Malaysia (.edu .my). Still others do not maintain a second-level domain specifically for academic institutions. In France, Germany, Switzerland, and the Netherlands, for example, each institution will have its own second-level domain (thus sorbonne .fr for the Sorbonne, hslu .ch for the Lucerne University of Applied Sciences and Arts and tum .de for the Technical University of Munich).
In some countries, both .edu and .ac second-level domains exist, differentiating between different types of academic institutions. China, for example, announced in 2006 that it would use .edu .cn for educational institutions and .ac .cn for research institutions.