Mark Liberman at Language Log establishes that the noun phrase ‘bright line’ and the adjective ‘bright-line’ are missing in most general English dictionaries:

bq. Here’s another common expression that’s not in the standard dictionaries: bright line, in the sense of “clear criterion of demarcation”.You won’t find it in the American Heritage Dictionary (fourth edition), Merriam-Webster’s Unabridged (third edition), Encarta, or the Oxford English Dictionary, but it’s widely used these days. Google has 193,000 hits for “bright line”, a substantial fraction of which seem to be instances of this expression. Searching today’s Google News brings up 104 examples, and Technorati finds 20 blog examples in the past three days.

As he says, you can work out the meaning from the Google examples.

The term is a U.S. one afaik. It’s in Black’s Law Dictionary, though only since the 7th ed., i.e. post-Garner. It’s in Merriam Webster’s Dictionary of Law. I can’t find it in Gifis (Barron’s) Law Dictionary, though. And I can’t find it in any bilingual law dictionaries.

bq. (Though Merriam-Webster’s Dictionary of Law, 1996 edition, has no entry for this term.)

Hmm.

Still, it can be found. German general dictionaries have a special rule of not allowing specialist terminology in. So the German equivalent of ‘bright line’, if there were one, might be found in a German law dictionary like Creifelds or Alpmann-Schmidt, but some verbs and adjectives with a specific use in legal texts won’t be found anywhere. To say nothing of Austrian and Swiss terminology that you can’t even find in Austrian and Swiss law dictionaries.

I have now found an entry in Garner’s Dictionary of Modern Legal Usage:

bq. bright-line rule = a judicial rule of decision that is simple and straightforward and that avoids or ignore the ambiguities or difficulties of the problems at hand. The phrase dates from the mid-20th century. The metaphor of a bright line is somewhat older than the phrase bright-line rule – e.g.: “The difficult part of this case comes with regard to … the activity of the Board of Temperance ….A bright line between that which brings conviction to one person and its influence on the body politic cannot be drawn.” Girard Trust Co. v. I.R.C., 122 F.2d 108, 110 (3d Cir. 1941)./”[T]he McCambridge majority opinion … agrees that the Kirby bright-line-rule is but a mere formalism ….” J.G.Trichter, Bright-Lining Away the Right to Counsel, Tex. Law., 6 Nov. 1989, at 26. Cf. hard and fast rule.