Ordinary Time is the liturgical period outside of the distinctive liturgical seasons, and runs 33 or 34 weeks. In Latin, Ordinary Time is called Tempus Per Annum ("time throughout the year"). Ordinary Time falls between Christmas and Lent, and between Easter and Advent, exclusive.
Isn't Ordinary Time just... well... ordinary?
Not at all. Ordinary Time is not the mundane, plain, humdrum part of the year. There are many feast days and celebrations that happen during this time od the year that make it quite dynamic. The music used at Mass might not be characteristically solemn (Lent), joyful (Easter), anticipating (Advent) or festive (Christmas), but it is all of those and more.