Literally, a person born within earshot of
the bells of St Mary-le-Bow church in what is now the City of London. However, technically speaking there can be no cockneys born after 1945 since the bells were destroyed by German bombs during WWII.
Today the term has expanded to
encompass not only those from
East London specifically but from London in general. The latter tends to be attributed by non-Londoners, stemming from their ignorance of the true meaning of the word.