middle has 10 distinct synonyms spread across 4 of its 9 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. Its 4 recorded opposites are end, beginning, late, early.
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.
an area that is approximately central within some larger region
“it is in the center of town”
the middle area of the human torso (usually in front)
“young American women believe that a bare midriff is fashionable”
being neither at the beginning nor at the end in a series
“adolescence is an awkward in-between age”
of a stage in the development of a language or literature between earlier and later stages
“Middle English is the English language from about 1100 to 1500”
Synonyms, antonyms and the broader and narrower relations for middle come from Princeton University’s WordNet. Full sources and licences.