The word triennium is defined as a period of three years. There are various synonyms for this word including triad, triptych, triquetra, trine, trilogy, and triad. Each of these words carries a slightly different connotation. For example, triptych often refers to an artwork that is split into three panels, while triquetra is a Celtic symbol commonly used to represent the Holy Trinity. Trilogy, on the other hand, is a term often used in literature and refers to a series of three books. Regardless of the synonym used, these words all convey the same basic meaning - a time period of three years.