A name applied to varying parts of the Balkans over the past two millenia. It has generally referred to portions of the eastern coast of the Adriatic Sea, stretching further inland according to the local influence of its rulers and the whims of external ones (including Rome and Vienna).
It is best known in English for its use in Twelfth Night, where Viola asks,
What country, friends, is this?
to which the captain replies,
This is Illyria, my lady.