A type of particle with "integral angular momentum" — a spin of 0, 1, 2, etc. Spin refers to an intrinsic quality of all particles. Examples of fermions are photons (which are the particles which give us light) and gravitons (which give us gravity). The other type of particle is the fermion.
Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.
The Red Queen in Lewis Carroll's Through the Looking Glass