"very large diurnal raptorial bird of the genus Aquila," mid-14c., from Old French egle, from Old Provençal aigla, from Latin aquila "black eagle," fem. of aquilus "eagle," often explained as "the dark colored" (bird); see aquiline. The native term was erne.
Golf score sense is by 1908 (according to old golf sources, because it "soars higher" than a birdie). As the name of a U.S. $10 coin minted from 1792 to 1933, established in the 1786 resolution for a new monetary system (but at first only the desperately needed small copper coins were minted). The figurative eagle-eyed "sharp-sighted" (like an eagle) is attested from c. 1600.