Post Roman Britain

The sense of isolation felt by the Welsh Celts or "Britons" was hastened by the Saxon victories that cut off their fellow Britons in Cornwall to the South and Cumbria to the North. From the momentous year 616, the date of the Battle of Chester, the Welsh people in Wales were on their own. Separated from their fellow Celts, those who lived west of Offa's Dyke gradually began to think of themselves as a distinct nation in spite of the many different rival kingdoms that developed within their borders such as Morgannwg, Powys, Brycheinion, Dyfed and Gwynedd. It is also from this period that we can speak of the Welsh language, as distinct from the older Britonic. The literary merits of the "new" language was amply and admirably demonstrated in the late seventh century poetry of Taliesin and Aneirin (though these were composed in the northern Celtic kingdoms of what is now Strathclyde, Scotland).
In a poem dated 633, the word Cymry appears, referring to the country; its use shows a self-awareness among the Britons, and it was not too long before they themselves came to be known as the Cymry, by which term they are known today. At this point, we should point out that the word Welsh is a later word used by the Saxon invaders of the British Isles perhaps to denote people they considered "foreign" or at least to denote people who had been Romanized. It originally had signified a Germanic neighbor, but eventually came to be used for those people who spoke a different language. The Welsh people themselves still prefer to call themselves "Cymry," their country "Cymru," and their language "Cymraeg."