A leaf is the flat, usually green part of a plant that grows from a stem and uses sunlight to make food; it can also mean a sheet of paper in a book.
From Old English *lēaf*, meaning leaf or foliage, related to German *Laub*. The sense of a sheet of paper came from its flat, thin shape, like a plant leaf.
The idea of a *leaf* jumped from plants to paper, so a “leaf of a book” is literally a paper leaf. When you “turn over a new leaf,” you’re metaphorically flipping to a fresh, blank page in your life.
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