1670s as a technical term in perspective drawing; more generally by 1706 as "the representation of anything drawn on a plane; a drawing, sketch, or diagram of any object," from French plan "ground plot of a building, map," literally "plane surface" (mid-16c.), from Latin planum "level or flat surface," noun use of adjective planus "level, flat" (from PIE root *pele- (2) "flat; to spread").
The notion is of "a drawing on a flat surface." A doublet of plain via a later, learned French form. The meaning "scheme of action, formulated scheme for the accomplishment of some object or attainment of an end" is by 1713.
1728, "make a plan of; put on paper the parts, dimensions, and methods of construction of," from plan (n.). By 1737 as "to scheme, to devise ways and means for (the doing of something)." Related: Planned; planning; plans. Planned economy is attested by 1931. Planned Parenthood (1942) formerly was Birth Control Federation of America.
1716, "one who plans, a deviser or arranger," agent noun from plan (v.). Betjeman coined the derogatory variant planster in 1945. Meaning "book or device that enables one to plan" is from 1971.
late Old English plot "small piece of ground of defined shape," a word of unknown origin. The sense of "ground plan," and thus "map, chart, survey of a field, farm, etc." is from 1550s. Plat is a Middle English collateral form. The meaning "a secret, plan, fully formulated scheme" (usually to accomplish some evil purpose) is from 1580s, probably by accidental similarity to complot, from Old French complot "combined plan" (compare the sense evolution of plan), itself a word of unknown origin, perhaps a back-formation from compeloter "to roll into a ball," from pelote "ball." OED says "The usage probably became widely known in connexion with the 'Gunpowder Plot.' "
The meaning "set of events in a story, play, novel, etc." is from 1640s. Plot-line (n.) "main features of a story" is attested by 1940; earlier, in theater, "a sentence containing matter essential to the comprehension of the play's story" (1907).
1540s, "plan of action, scheme, design;" 1550s, "ground-plan, drawing, sketch," senses now obsolete, from French plateforme, platte fourme, literally "flat form," from Old French plat "flat, level" (see plateau (n.)) + forme "form" (see form (n.)). These senses later went with plan (n.).
The sense of "raised, level surface or place" in English is attested from 1550s, especially "raised frame or structure with a level surface." Specifically in geography, "flat, level piece of ground," by 1813. The railroad station sense of "raised walk along the track at a station for landing passengers and freight" is from 1832.
The U.S. political meaning, "statement of political principles and of the course to be adopted with regard to certain important questions of policy, issued by the representatives of a political party assembled in convention to nominate candidates for an election," is from 1803. It is probably originally an image of a literal platform on which politicians gather, stand, and make their appeals, and perhaps it was influenced by the earlier sense in England of "set of rules governing church doctrine" (1570s). In 19c., platform was used generally in a figurative sense for "the function of public speaking," and even was a verb, "to address the public as a speaker."
It forms all or part of: airplane; dysplasia; ectoplasm; effleurage; esplanade; explain; explanation; feldspar; field; flaneur; floor; llano; palm (n.1) "flat of the hand;" palm (n.2) "tropical tree;" palmy; piano; pianoforte; plain; plan; planar; Planaria; plane (n.1) "flat surface;" plane (n.3) "tool for smoothing surfaces;" plane (v.2) "soar, glide on motionless wings;" planet; plani-; planisphere; plano-; -plasia; plasma; plasmid; plasm; -plasm; -plast; plaster; plastic; plastid; -plasty; Polack; Poland; Pole; polka; protoplasm; veldt.
It is the hypothetical source of/evidence for its existence is provided by: Greek plassein "to mold," plasma "something molded or created;" Latin planus "flat, level, even, plain, clear;" Lithuanian plonas "thin;" Celtic *lanon "plain;" Old Church Slavonic polje "flat land, field," Russian polyi "open;" Old English feld, Middle Dutch veld "field."