a. Having very keen vision; sharp-sighted; discerning. [ wns=1 ] Syn. -- keen-sighted, lynx-eyed, quick-sighted, sharp-eyed, sharp-sighted. [ 1913 Webster ]
2. alert to possible danger. [ wns=2 ] Syn. -- argus-eyed, open-eyed, unsleeping, vigilant, wary, watchful. [ WordNet 1.5 ]