From Wikipedia—Ordinary Time is a season of the Christian (especially the Catholic) liturgical calendar. The English name is intended to translate the Latin term Tempus per annum (literally "time through the year"). Ordinary Time comprises the two periods — one following Epiphany, the other following Pentecost — which do not fall under the "strong seasons" of Advent, Christmas, Lent, or Easter.
How do we get "Ordinary Time" from "time through the year"? I didn't realize that this is one of the unintended consequences of Vatican II. I think a better explanation is given here.