The Magic School Bus is the titular overarching protagonist of the series of the same name.
It is the anthropomorphic school bus that Ms. Frizzle's class and Liz ride on and goes on field trips. It is most often piloted by Ms. Frizzle, her sister Ms. Frizzle or Liz, but occasionally, one of the kids will take the wheel, or it will take on a shape (such as an animal) that allows it to "drive" itself.