A decrease in the density of something.

Roget 194: expansion; increase of size etc. 35; enlargement, extension, augmentation; amplification, ampliation^†; aggrandizement, spread, increment, growth, development, pullulation, swell, dilation, rarefaction; turgescence^†, turgidness, turgidity; dispansion^†; obesity etc. (size) 192; hydrocephalus, hydrophthalmus [Med.]; dropsy, tumefaction, intumescence, swelling, tumor, diastole, distension; puffing, puffiness; inflation; pandiculation^†.    dilatability, expansibility.    germination, growth, upgrowth^†; accretion etc. 35; budding, gemmation^†.    overgrowth, overdistension^†; hypertrophy, tympany^†.    bulb etc. (convexity) 250; plumper; superiority of size.    [expansion of the universe] big bang; Hubble constant.    ... show more

Roget 322: rarity, tenuity; absence of solidity etc. 321; subtility^†; subtilty^†, subtlety; sponginess, compressibility.    rarefaction, expansion, dilatation, inflation, subtilization^†.    vaporization, evaporation, diffusion, gassification^†.    ether etc. (gas) 334.    ... show more

Dutch: rarefactie