What's the difference between i.e. and e.g.?
1660s, with -an + Latin caeruleus "blue, dark blue, blue-green," perhaps dissimilated from caelulum, diminutive of caelum "heaven, sky," of uncertain origin (see celestial). The Latin word was applied by Roman authors to the sky, the Mediterranean, and occasionally to leaves or fields. As a noun, from 1756.