Of elements, features, attributes, etc.: sharp, piercing, keen. Obsolete.

Acute or penetrating in intellect or perception. Hence of observation, an observer: Vigilant. Phrase, to keep a sharp look-out.

Of the eyes, etc.: critical, searching.

Keenly observant; acutely perceptive, penetrating; (in later use often of a look or gaze) that focuses intently or probingly on someone or something.

That scans or examines closely; critical, searching.