Depends on context.

Amongst concertina players, it means a particular system, or arrangement of buttons.

Amongst languages, it means the result of mixing German and French together, with pinches of Latin, Greek, Welsh and other languages to taste, and simmering for a thousand years.

In many parts of the world, it is considered to be synonymous with 'British', which isn't strictly true - many of us are British but not English.

Geographically, it is the adjectival derivation of that part of Britain bordered (approximately) by Hadrian's Wall and Offa's Dyke.

In what context did you pose the question?