St Thomas Becket, (21 December c. 1119 (or 1120) – 29 December 1170), ambitious Chancellor to Henry II, politically appointed Archbishop of Canterbury to Henry II, and martyr (thanks to Henry II). Becket was a fairly low-born cleric who rose to be Henry II's right hand man, before being created Archbishop of Canterbury in 1162; this was an attempt by Henry to extend his control into the Church, expecting the infamously worldly and obsequious Becket to do his bidding without question. Unexpectedly, Becket apparently became extremely devout, refusing to comply with the King's meddling in religious affairs. After many disputes, often over quite petty issues, some as minor as whether the Archbishop of York should carry his own cross or not, Becket was murdered in Canterbury cathedral shortly after returning from exile, by four knights who believed that they were carrying out Henry's instructions, in history's most notorious Rhetorical Request Blunder. Traditionally, the king is said to have lamented "will no one rid me of this Turbulent Priest", but the most widely accepted account has him uttering the far less ambiguous "What miserable drones and traitors have I nourished and brought up in my household, who let their lord be treated with such shameful contempt by a low-born cleric?"
Overnight, Becket became a martyr of European renown, was canonized within three years of his death, and remained England's most venerated saint until the Reformation. Ironically, Becket would have the last laugh, as Henry was forced to abandon his attempts to wrest control of the church both to appease Pope Alexander III and to combat his reputation as a blasphemer among his unhappy subjects and disgruntled diplomatic allies. His tomb in Canterbury was a popular pilgrimage spot in the rest of the medieval era, as chronicled by Geoffrey Chaucer's The Canterbury Tales, until it was dismantled under the orders of Henry VIII during the English Reformation.
You may see his name written with a gratuitous "à" as Thomas à Becket. This is a particularly odd case of the Adaptation Name Change, due to a 16th-century fad among historians of inserting the à to make their subjects seem like folk heroes. When the fad died out, Thomas' new name had entered more general usage (perhaps it rolls off the tongue quite well), but seeing it after about 1920 probably means the author hasn't quite caught up with the times.
The subject of many paintings, hagiographies, biographies, plays, and films, Becket is most notable as the subject of Anouilh's play, later a film, Becket, and T. S. Eliot's play Murder in the Cathedral.