shorten has 10 distinct synonyms spread across 2 of its 5 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 one recorded opposite is lengthen.
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.
reduce in scope while retaining essential elements
“The manuscript must be shortened”
Synonyms, antonyms and the broader and narrower relations for shorten come from Princeton University’s WordNet. Full sources and licences.