Ordinary Time refers to all of those parts of the Catholic Church's liturgical year that aren't included in the major seasons of Advent, Christmas, Lent, and Easter. Ordinary Time thus encompasses two different periods in the Church's calendar, since the Christmas season immediately follows Advent, and the Easter season immediately follows Lent.
The Church year begins with Advent, followed immediately by the Christmas season. Ordinary Time begins on the Monday after the first Sunday after January 6, the traditional date of the Feast of the Epiphany and the end of the liturgical season of Christmas. This first period of Ordinary Time runs until Ash Wednesday when the liturgical season of Lent begins. Both Lent and the Easter season fall outside of Ordinary Time, which resumes again on the Monday after Pentecost Sunday, the end of the Easter season. This second period of Ordinary Time runs until the First Sunday of Advent when the liturgical year begins again.