bright-line

Merriam-Webster's Word of the Day for January 28, 2012 is:

bright-line • \BRYTE-lyne\  • adjective
: providing an unambiguous criterion or guideline especially in law

Examples:
The company's new reimbursement policy makes a bright-line distinction between acceptable and unacceptable travel expenses.

"The NFL needs to have a bright-line rule for the use of electronics devices during games." -- From a post by Mike Florio at nbcsports.com, January 13, 2012

Did you know?
In the first half of the 20th century, courts began referring to a "bright line" that could or could not be drawn to make clear-cut distinctions between legal issues. Early users may have been influenced by the term "bright line," used by physicists to refer to the distinct color lines in the light spectrum. Before that, judges were content with wording that was more prosaic, such as "line of demarcation." In the second half of the 20th century, we began using "bright-line" as an adjective. Nonlegal types looking for unambiguous distinctions in other walks of life took a shine to "bright-line" sometime in the 1980s.