Noun
A small, common warbler, Phylloscopus collybita, with yellowish-green plumage that breeds throughout northern and temperate Europe and Asia.
Any of several other species of the same genus.
(UK, onomatopoeic, uncountable) The song of the chiffchaff.
I thought I heard something singing chiffchaff, chiffchaff. So I had a look and there was a chiffchaff.