A link from a .edu domain (such as harvard.edu).
.edu is the top-level domain reserved for use by institutions of higher education within the U.S. Due to the affiliation with educational institutions links from .edu domains may be given more weight by search engines in their ranking algorithms.
Note: Prior to 2001 the requirements for registering a .edu domain were less restrictive and not strictly enforced. A small number of .edu domains not registered to institutions of higher education are still active, for example http://www.exploratorium.edu.