idiosyncrasy

Concise Oxford English Dictionary © 2008 Oxford University Press:
idiosyncrasy /ˌɪdɪə(ʊ)ˈsɪŋkrəsi/

▶noun (pl. idiosyncrasies)

• 1 a mode of behaviour or way of thought specific to an individual.

  ■ a distinctive or peculiar characteristic of something.

• 2 Medicine an abnormal physical reaction by an individual to a food or drug.

– derivatives
idiosyncratic adjective,
idiosyncratically adverb.

– origin C17: from Gk idiosunkrasia, from idios ‘own’ + sun ‘with’ + krasis ‘mixture’.