Etymology
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landmark (n.)

Old English landmearc "object set up to mark the boundaries of a kingdom, estate, etc.," from land (n.) + mearc in its sense "object which marks a boundary or limit" (see mark (n.1)). General sense of "conspicuous object which, by its known position, serves as a guide to a traveler," originally especially an object that can be seen from sea, is from 1560s. Modern figurative sense of "event, etc., considered a high point in history" is from 1859.

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*merg- 
Proto-Indo-European root meaning "boundary, border."

It forms all or part of: Cymric; demarcation; Denmark; emarginate; landmark; march (v.) "walk with regular tread;" march (n.2) "boundary;" marchioness; margin; margrave; mark (n.1) "trace, impression;" mark (n.2) "unit of money or weight;" marque; marquee; marquetry; marquis; remark; remarkable.

It is the hypothetical source of/evidence for its existence is provided by: Latin margo "margin;" Avestan mareza- "border;" Old Irish mruig, Irish bruig "borderland," Welsh bro "district;" Old English mearc "boundary, sign, limit, mark," Gothic marka "boundary, frontier."
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Marley 

surname, from place-names in England, probably "boundary wood or clearing," from Old English mære "boundary, border, landmark" (see merestone) + leah (see lea).

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mural (n.)

painting on a wall, by 1915, short for mural painting "a painting executed upon the wall of a building" (1850), from mural (adj.) "pertaining to a wall or walls" (mid-15c.), from Latin muralis "of a wall," from murus "wall" (Old Latin moiros, moerus), from PIE *mei- (3) "to fix; to build fences or fortifications" (source also of Old English mære "boundary, border, landmark;" Old Norse -mæri "boundary, border-land;" Latin munire "to fortify, protect").

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horizon (n.)
late 14c., orisoun, from Old French orizon (14c., Modern French horizon), earlier orizonte (13c.), from Latin horizontem (nominative horizon), from Greek horizon (kyklos) "bounding (circle)," from horizein "bound, limit, divide, separate," from horos "boundary, landmark, marking stones." The h- was restored in English 17c. in imitation of Latin. Old English used eaggemearc ("eye-mark") for "limit of view, horizon." The apparent horizon is distinguished from the celestial or astronomical horizon.
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mere (n.2)

"boundary line" (between kingdoms, estates, fields, etc.), now surviving in provincial use or place names, but once an important word, from Old English mære "boundary, object indicating a boundary," from Proto-Germanic *mairjo- (source also of Middle Dutch mere "boundary mark, stake," Old Norse -mæri "boundary, border-land"), related to Latin murus "wall" (see mural (n.)).

Hence merestone "stone serving as a landmark" (Old English mærstan); mere-stake "pole or tree standing as a mark or boundary" (1620s); meresman "man appointed to find boundaries" (of a parish, etc.). In Middle English meres of erthe (c. 1400) was "the ends of the earth."

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flora (n.)
c. 1500, "Roman goddess of flowers;" 1777, "the plant life of a region or epoch," from Latin Flora, "goddess of flowers," from flos (accusative florem, genitive floris) "flower," from *flo-s-, Italic suffixed form of PIE *bhle- "to blossom, flourish" (source also of Middle Irish blath, Welsh blawd "blossom, flower," Old English blowan "to flower, bloom"), extended form of root *bhel- (3) "to thrive, bloom."

Her festival, the Floralia, was April 28 to May 2 and featured "comic theatrical representations" and "excessive drinking" [Century Dictionary]. The French Revolutionary calendar had a month Floréal (April 20-May 20). Used as the title of systematically descriptive plant catalogues since 1640s, but popularized by Linnaeus in his landmark 1745 study of Swedish plants, "Flora Suecica."
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mark (n.1)

"trace, impression," Old English mearc (West Saxon), merc (Mercian) "boundary, limit; sign, landmark," from Proto-Germanic *markō (source also of Old Norse merki "boundary, sign," mörk "forest," which often marked a frontier; Old Frisian merke, Gothic marka "boundary, frontier," Dutch merk "mark, brand," German Mark "boundary, boundary land"), from PIE root *merg- "boundary, border." Influenced by, and partly from, Scandinavian cognates. The Germanic word was borrowed widely and early in Romanic (compare marque; march (n.2), marquis).

The primary sense "boundary" had evolved by Old English through "pillar, post, etc. as a sign of a boundary," through "a sign in general," then to "impression or trace forming a sign." Meaning "any visible trace or impression" is recorded by c. 1200. Meaning "a cross or other character made by an illiterate person as a signature" is from late Old English. Sense of "line drawn to indicate the starting point of a race" (as in on your marks..., which is by 1890) is attested by 1887.

The Middle English sense of "target" (c. 1200) is the notion in marksman and slang sense "victim of a swindle" (1883). The notion of "sign, token" is behind the meaning "a characteristic property, a distinctive feature" (1520s), also that of "numerical award given by a teacher" (by 1829). To make (one's) mark "attain distinction" is by 1847.

In medieval England and in Germany, "a tract of land held in common by a community," hence Mark of Brandenburg, etc.

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