Zeeman effect – the observed splitting of spectral lines into separate components by applying a magnetic field to the atom

Zero-point energy – the minimum energy a particle can have

Zeroth law of thermodynamics – states that two objects, each in thermal equilibrium with a third object, are in thermal equilibrium with each other

Zoom lens – a lens which can be changed quickly from a wide-angle lens (short focal length) to a telephoto lens (long focal length), maintaining its focus during the entire zoom range