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Old English landmearc, from land (n.) + mearc (see mark (n.1)). Originally "object set up to mark the boundaries of a kingdom, estate, etc.;" general sense of "conspicuous object in a landscape" is from 1560s. Modern figurative sense of "event, etc., considered a high point in history" is from 1859.
a boundary line indicated by a stone, stake, etc. (Deut. 19:14; 27:17; Prov. 22:28; 23:10; Job 24:2). Landmarks could not be removed without incurring the severe displeasure of God.