Definition of "unambiguous"

An LR(k) grammar is clearly unambiguous, since the definition implies every derivation tree must have the same handle, and by induction there is only one possible tree. It is interesting to point out furthermore that nearly every grammar which is known to be unambiguous is either an LR(k) grammar, or (dually) is a right-to-left translatable grammar, or is some grammar which is translated using "both ends toward the middle." Thus, the LR(k) condition may be regarded as the most powerful general test for nonambiguity that is now available.

1965 July, Donald Knuth, “On the Translation of Languages from Left to Right”, in Information and Control, volume 8, pages 639–707

At the height of the crisis, according to a retired SAC wing commander, SAC airborne alert bombers deliberately flew past their turnaround points toward Soviet airspace, an unambiguous threat which Soviet radar operators would certainly have recognized and reported. "I knew what my target was," the SAC general adds: "Leningrad." The bombers only turned around when the Soviet freighters carrying missiles to Cuba stopped dead in the Atlantic.

1995, Richard Rhodes, “Scorpions in a Bottle”, in Dark Sun: The Making of the Hydrogen Bomb‎, New York: Simon & Schuster, page 575