'arefaction' is used as a variant form related to 'rarefaction', ultimately from Latin 'rarificāre' (to make rare) formed from 'rārus' (sparse, thin) + 'facere' (to make).

'rarefaction' entered English via Medieval Latin and Old French forms (e.g. Medieval Latin 'rarifactio' / 'rarificatio'), and the variant spelling 'arefaction' has appeared historically as an alternative or analogical form to 'rarefaction'.

Originally related to the idea of 'making rare or thin', its modern meanings center on 'reduction of density' generally and specifically the low-pressure phase in longitudinal waves.