de Broglie wavelength
The de Broglie wavelength is the wavelength that quantum mechanics assigns to any moving piece of matter, given by a strikingly simple formula: the wavelength λ equals Planck's constant h divided by the particle's momentum p. Fast, heavy objects have a tiny momentum-to-h ratio, so their wavelength is absurdly small; slow, light objects like electrons can have wavelengths comparable to the spacing of atoms, which is why their wave nature shows up in experiments.
This single relation ties the particle world and the wave world together with one knob, momentum. Push a particle to higher momentum and its wavelength shrinks; let it slow down and the wavelength grows. It is the matter-side mirror of an idea Einstein had already used for light, where a photon's momentum and wavelength obey the very same kind of relationship.
The reason you never notice the wavelength of a thrown baseball is that Planck's constant is fantastically small, so the ball's de Broglie wavelength is many trillions of times smaller than an atomic nucleus — utterly undetectable. For an electron in an atom, by contrast, the wavelength is roughly the size of the atom itself, and that match is exactly what makes the atom a quantum object rather than a miniature solar system.
Bigger momentum means shorter wavelength; only very light, slow things have wavelengths large enough to notice.
The de Broglie wavelength is not the size of the particle, nor a literal ripple travelling through space. It sets the scale on which interference and diffraction become visible, and it appears as the wavelength of the particle's wavefunction.