Quinquagesima is the former name for the Sunday before Ash Wednesday. It was also called Quinquagesima Sunday, Shrove Sunday or Esto Mihi. The name originates from Latin quinquagesimus (fiftieth), referring to the fifty days before Good Friday (this calculation includes Sundays, unlike the quadragesimus calculation for Ash Wednesday itself).
These terms began to fall out of use after the Second Vatican Council dropped this Sunday (and the two immediately before it - Sexagesima and Septuagesima Sundays) from the Roman Catholic liturgical calendar effective in 1970, with the Anglican churches doing likewise six years later; as a consequence, today this Sunday is now known by its number within Ordinary Time - fourth through ninth, depending upon the date of Easter - in the Roman Catholic calendar, or the fourth through the ninth Sunday After Epiphany in the Anglican calendar, and that of many other Protestant polities.
A quinquagesima can also be any fifty day period in the Catholic year, such as the time from Easter to Pentecost.