The NCAA rulebook defines eligible receivers for college football in Rule 7, Section 3, Article 3.
[2] A receiver loses his eligibility by leaving the field of play unless he was forced out by a defensive player and immediately attempts to get back inbounds (Rule 7–3–4).
In both American and Canadian professional football, every player on the defensive team is considered eligible.
In most cases where a pass is caught by an ineligible receiver, it is usually because the quarterback was under pressure and threw it to an offensive lineman out of desperation.
An example of this was a 1985 NFL game in which William Perry, wearing number 72 and normally a defensive lineman, was made an eligible receiver on an offensive play, and successfully caught a touchdown pass attempt.
[5] In the 2019 season the Atlanta Falcons declared right tackle Ty Sambrailo eligible on many plays before throwing the ball to him for a 35-yard touchdown against the Tampa Bay Buccaneers.
In the CFL all players become eligible receivers if a pass is touched by a member of the defensive team.
A proposed rule change in the XFL would make all players behind the line of scrimmage eligible receivers, regardless of position or number.
This was because of a change in the definition of a scrimmage-kick formation made by the National Federation of State High School Associations (NFHS).