Genesis is a rather serious, book-like word for the beginning of something — not just the first day, but the origin, the moment or process from which it grew. We say the genesis of an idea, the genesis of a war, the genesis of a company. You would not use it for “the start of class” or “the beginning of the film”; that would sound too heavy. It belongs to writing, talks, and careful speech. In ordinary talk, beginning or origin will usually do; genesis adds a slow, slightly scientific or literary colour. Though actually, many students meet the word first as a name: Genesis, the first book of the Bible, which tells a story of how the world began. That name and this meaning are the same word — the book is called that because it is about a beginning. People also use Genesis as a proper name for a person, a band, or a car. Do not mix it up with genetics, which is about genes in living things; they sit near each other on the page, but they are not the same tool.