Coastline is from coast plus line. The formation is semantically transparent: the parts remain visible, while the compound or affix fixes the grammatical role and narrows the modern meaning. In use as a noun, coastline denotes the outline or shape of a coast where the land meets the sea. Typical contexts include “The rugged coastline stretched for miles”. The meaning stays close to the form: it applies the etymological material to the outline or shape of a coast where the land meets the sea, keeping the visible parts legible in ordinary use.