Trin’ity. Tertullian (160-240) introduced this word into Christian theology. The word triad is much older. Almost every mythology has a threefold deity. 1
There are three terms at Cambridge in a year, and four at Oxford, but the two middle Oxford terms are two only in name, as they run on without a break. The three Cambridge terms are Lent, Easter, and Michaelmas. The four Oxford terms are Lent, Easter + Trinity, and Michaelmas.