Quinquagesima literally means "fiftieth." It is 49 days before Easter, or 50 if you count Easter itself. (Similarly, Pentecost Sunday is said to be 50 days after Easter, but the number is calculated by including Easter in the count.)
When the Roman Catholic liturgical calendar was revised in 1969, the three pre-Lenten Sundays were removed; they are now denominated simply as Sundays in Ordinary Time. Septuagesima, Sexagesima, and Quinquagesima are all still observed in the celebration of the traditional Latin Mass.
For more information about Quinquagesima and the other pre-Lenten Sundays, see "What Is Septuagesima Sunday?"