reduce has 33 distinct synonyms spread across 12 of its 20 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 2 recorded opposites are blow up, gain.
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.
make less complex
“reduce a problem to a single question”
bring to humbler or weaker state or condition
“He reduced the population to slavery”
simplify the form of a mathematical equation of expression by substituting one term for another
lower in grade or rank or force somebody into an undignified situation
“She reduced her niece to a servant”
lessen and make more modest
“reduce one's standard of living”
to remove oxygen from a compound, or cause to react with hydrogen or form a hydride, or to undergo an increase in the number of electrons
reposition (a broken bone after surgery) back to its normal site
destress and thus weaken a sound when pronouncing it
reduce in scope while retaining essential elements
“The manuscript must be shortened”
be cooked until very little liquid is left
“The sauce should reduce to one cup”
cook until very little liquid is left
“The cook reduced the sauce by boiling it for a long time”
take off weight
Synonyms, antonyms and the broader and narrower relations for reduce come from Princeton University’s WordNet. Full sources and licences.