Merriam-Webster's Collegiate Dictionary
tril·o·gy
\\ˈtri-lə-jē\\ noun
(plural -gies) ETYMOLOGY Greek trilogia, from tri- + -logia -logy
DATE circa 1661
: a series of three dramas or literary works or sometimes three musical compositions that are closely related and develop a single theme
trilogy
1661, "any series of three related works," from Gk. trilogia "series of three related tragedies performed at Athens at the festival of Dionysus," from tri- "three" + logos "story."
Oxford Advanced Learner's Dictionary-牛津双解-OALD7
trilogy
tril·ogy / 5trilEdVi / noun (pl. -ies)
a group of seven books, films / movies, etc. that have the same subject or characters
(书籍、电影等的)三部曲；三部剧
Webster's Third New International Dictionary, Unabridged
tril·o·gy
\ˈtriləjē, -ji\ noun
(-es)
Etymology: Greek trilogia, from tri- + -logia -logy
1. : a series of three dramas or sometimes three literary or musical compositions that although each is in one sense complete have a close mutual relation and form one theme or develop aspects of one basic concept
2. : a group of three connected classical Greek tragedies played serially (as at the festival of Dionysus)
3. : a group of three related things, topics, or sayings
: triad