Veil law was the set of rules instituted in the mid-80's that states that boys cannot be in girls' halls, and girls cannot be in boys' halls, for *cough* obvious reasons. Prior to these rules, there was simply an open-door policy, where boys and girls were allowed to be in the same room so long as the door was open. These rules were collectively known as the Veil law, because the theory was that next the administration was going to make the girls wear veils. Eventually, this rule was accepted as a matter of course.
This rule exists at all sites and is rarely referred to by this name. It is commonly placed under scrutiny by students because of the double negative referring to the fact that a "DON'T" is "No males on female halls," and another is "No females on male halls," leading students to say that if there is not at least one male on a female hall, a rule is being broken.
Bram Boroson coined the term at Carlisle in 1984 (think Orwell).