The beginning is the first part or section of something, or the place where it starts. You watch the opening credits at the beginning of a movie.
"In the beginning," says the beginning of the Bible, "God created the heavens and the earth." Beginning can mean the act of starting something (the day after Thanksgiving is the beginning of the Christmas shopping season), or it can mean the time when something will start (Beginning at noon, all Christmas ornaments will go on sale.) Beginning can also refer to the origin of something, its root or source, as the beginning of the Mississippi River is Lake Itasca, Minnesota.