a common short European plant bearing flowers with yellow centres and white rays with a yellow disc and white or pink ray flowers: Bellis perennis
used in the names of various other similar plants, e.g. the Michaelmas daisy or oxeye daisy
pushing up (the) daisies
informal dead and in one's grave
[Old English dægesēage, from dæg day + ēage eye1; because the flower opens in daylight]