Line positions including center, guard and offensive tackle are not eligible receivers. (A tackle may be eligible if a team lines up with an unbalanced line.)
See also: Glossary of American football
Eligible Receiver was the name of a top-secret United States National Security Agency operation taking place in summer 1997. A world-class team of hackers tested the Pentagon's cyber-defences, using only tools downloaded for free off of the Internet. The results of it were appalling. The hackers had, essentially, free rein over many portions of the network. Information about the operation became public in the next year.
See also: Hacking