An atom laser is a hypothetical device which would produce an atomic field analogous to the electromagnetic field of a photon laser. Here I argue that for this analogy to be meaningful it is necessary to have a precise definition of a laser which applies equally to photon or atom lasers. The definition I propose is based upon the principle that the output of a laser is well-approximated by a classical wave of fixed intensity and phase. This principle yields four quantitative conditions which the output of a device must satisfy in order for that device to be considered a laser. While explaining these requirements, I analyse the similarities and differences between atom and photon lasers. I show how these conditions are satisfied first by an idealized photon laser model, and then by a more generic model which can apply to atom lasers also. Lastly, I briefly discuss the current proposals for atom lasers and whether they could be true lasers.