backlog has 2 distinct synonyms spread across 1 of its 4 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.
an accumulation of jobs not done or materials not processed that are yet to be dealt with (especially unfilled customer orders for products or services)
the large log at the back of a hearth fire
Synonyms, antonyms and the broader and narrower relations for backlog come from Princeton University’s WordNet. Full sources and licences.
something kept back or saved for future use or a special purpose