floor has 14 distinct synonyms spread across 6 of its 12 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the inside lower horizontal surface (as of a room, hallway, tent, or other structure)
“they needed rugs to cover the bare floors”
a lower limit
“the government established a wage floor”
the ground on which people and animals move about
“the fire spared the forest floor”
the bottom surface of any lake or other body of water
the lower inside surface of any hollow structure
“the floor of the pelvis”
the occupants of a floor
“the whole floor complained about the lack of heat”
the parliamentary right to address an assembly
“the chairman granted him the floor”
the legislative hall where members debate and vote and conduct other business
“there was a motion from the floor”
a large room in a exchange where the trading is done
“he is a floor trader”
surprise greatly; knock someone's socks off
“I was floored when I heard that I was promoted”
Synonyms, antonyms and the broader and narrower relations for floor come from Princeton University’s WordNet. Full sources and licences.