the outline of objects seen against the sky
"The city's skyline was silhouetted in black against the bright orange sunset."
the line at which the sky and Earth appear to meet
"From atop the hill, we watched as the sun set behind the city's jagged skyline."
The line at which the earth and sky meet.
"The photographer waited until sunset to capture the perfect silhouette where the earth and sky met in a soft golden glow."
In plain English: A skyline is the outline of buildings and other tall structures seen against the sky from far away.
"The city's skyline was illuminated by thousands of lights after sunset."
Usage: The term refers to the visible contour of a city's buildings rather than the horizon where land meets sky in natural landscapes. It is often confused with "horizon," but skyline specifically implies an urban or built environment silhouette.
To outline something against the sky.
"The jagged silhouette of the city skyline outlined the mountains against the twilight sky."
In plain English: To skyline something means to make it look as if it is reaching up into the sky by adding tall parts to its top.
"The skyscrapers have finally begun to skyline the city horizon with their new heights."
The word skyline is a compound of sky and line that was first used to describe the outline formed by buildings against the horizon. It entered English as a straightforward combination of two existing words rather than through borrowing or shifting meanings.