concentration has 9 distinct synonyms spread across 3 of its 7 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 distribution, dilution.
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 strength of a solution; number of molecules of a substance in a given volume
strengthening the concentration (as of a solute in a mixture) by removing diluting material
increase in density
complete attention; intense mental effort
great and constant diligence and attention
Synonyms, antonyms and the broader and narrower relations for concentration come from Princeton University’s WordNet. Full sources and licences.