Merthyr Tydfil ( Welsh: Merthyr Tudful) is a town in Wales, with a population of 58,802 according to the 2011 UK census. It is situated approximately 23 miles (37 km) north of Cardiff. Once the largest town in Wales, it is now the 10th largest urban area in Wales. It is in the historic county of Glamorgan and is currently the main town in Merthyr Tydfil County Borough administered by Merthyr Tydfil County Borough Council. Both the town and the county borough are often referred to as 'Merthyr'.
According to legend, the town is named after Saint Tydfil, a daughter of King Brychan of Brycheiniog. According to her legend, she was slain at Merthyr by pagans around 480; the place was subsequently named Merthyr Tydfil in her honour. Although merthyr generally means "martyr" in modern Welsh, the meaning here is closer to the Latin martyrium: the mausoleum or church built over the relics of a martyr. Similar examples, all from south Wales, include Merthyr Cynog, Merthyr Dyfan and Merthyr Mawr. The Cornish and Breton language equivalents, in place names, are merther and merzher.
For more information, see the EN Wikipedia article Merthyr Tydfil. This is a long article. Family historians looking for a fairly brief outline should start at the heading "Early modern Merthyr".
The first three maps are provided by A Vision of Britain through Time
These maps were found on Wikimedia Commons
These maps of Glamorgan post-1974 were found on another site and are very useful for sorting out the up-to-date geography of the area