means The outline of buildings, hills, or other structures seen against the sky, especially the distinctive profile of a city's tall buildings on the horizon.

from A transparent compound of 'sky' and 'line,' English to its bones. 'Sky' comes from Old Norse 'ský,' meaning 'cloud' — the Vikings gave English the very word for the heavens. 'Line' traces back through Old English to Latin 'linea,' a string or thread of flax. The word first meant simply the line where earth or sea meets sky — the horizon — and only later, as cities grew tall enough to scrawl their own jagged edge across that horizon, did 'skyline' come to mean the silhouette of a metropolis.