'defervescence' originates from Latin, specifically the word 'defervescere', where 'de-' meant 'down, away' and 'fervescere' meant 'to grow hot' (intransitive form related to 'fervere', to boil).

'defervescere' passed into New Latin/medical Latin as 'defervescentia' or similar formations and was adopted into English as 'defervescence' in medical contexts.

Initially it referred generally to 'growing less hot' or 'ceasing to be hot'; over time it became specialized to mean 'the abatement of fever' and, by extension, a decline in intensity.