Catherine Swynford (c.1350-1403) was the mistress, then wife, of John of Gaunt and sister of the wife of Geoffrey Chaucer. She was married to a knight in John's retinue, and became John's mistress after the death of his wife, Blanche of Lancaster in a recurrence of the plague in 1368. She was governess to John's children from his second marriage - to Constance of Castile, and bore him children of her own, who were given the name Beaufort from one of his insignificant French estates. They were legitimised by Parliament after her marriage to John following Constance's death in 1396 and became a powerful and politically active noble line.
Henry VII’s mother was a Beaufort.
Reference: "The Oxford Companion to British History".