Category: Distinctions routinely treated as synonyms

The list below is believed complete, though membership — like everything here — is subject to revision.

Flat — Of no curvature anywhere — a plane. A stronger condition than 'level' or 'horizontal': a tilted floor is flat but not level, and still water is level but not flat.
Horizontal — The plane perpendicular to the plumb line — flat, and tangent to the level surface, but true only where it is taken; two distant horizontals are not parallel.
Level — The geodetic sense of the word: a surface everywhere perpendicular to gravity, and therefore — over any distance — curved. Still water is the demonstration.
A. Sterling Sill — Victorian gentleman (1821–1879) of independent means and a single proposition — that the horizontal does not slope — which is locally true, globally fatal, and as far as he ever got.