'Platform' is French for 'flat form' — originally a military term for a level gun emplacement.
A raised level surface on which people or things can stand; the declared policy of a political party; a computing or technology infrastructure on which applications run.
From French 'plate-forme' (flat form, ground plan, flat surface), a compound of 'plat' (flat) + 'forme' (form, shape, plan). The French 'plat' derives from Old French 'plat,' from Vulgar Latin *'plattus,' borrowed from Greek 'platys' (broad, flat), which traces to PIE *pleth2- (to spread out, to be flat), the same root that produced Latin 'planus' (flat, plain), English 'floor' (via Germanic *floruz), Sanskrit 'prthu' (broad), and Welsh 'llawr' (floor). The French 'forme' descends from Latin 'forma' (shape
The phrase 'party platform' in American politics dates from the 1830s, when political conventions began issuing formal statements of policy. The metaphor is literal: the platform was the raised stage from which candidates spoke, and their promises became the 'platform' they stood on.