The term comes not from the English word "ordinary" but rather from the term "ordinal" meaning numbered, and came into usage following the Second Vatican Council.
Ordinary Time is in two periods, one following Epiphany, the other following Pentecost. It includes and concludes with the festival of Christ the King. During this time, certain other festivals that fall on particular dates, rather than necessarily on a Sunday, may pre-empt the lectionary readings whenever they do fall on a Sunday. Whether this occurs, and exactly which festivals do pre-empt the lectionary, varies from tradition to tradition.