A Germ

1. (biology) The small mass of cells from which a new organism develops; a seed, bud or spore.
2. A pathogenic microorganism.
3. The embryo of a seed, especially of a seed used as a cereal or grain. See Wikipedia article on cereal germ.
4. (figurative) The origin of an idea or project.
5. (mathematics) An equivalence class that includes a specified function defined in an open neighborhood.