In quantum field theory, the mass gap is the difference in energy between the lowest energy state, the vacuum, and the next lowest energy state. The energy of the vacuum is zero by definition, and assuming that all energy states can be thought of as particles in plane-waves, the mass gap is the mass of the lightest particle.

Since the energies of exact (i.e. nonperturbative) energy eigenstates are spread out and therefore are not technically eigenstates, a more precise definition is that the mass gap is the greatest lower bound of the energy of any state which is orthogonal to the vacuum.

The analog of a mass gap in many-body physics on a discrete lattice arises from a gapped Hamiltonian.

For a given real-valued quantum field ${\displaystyle \phi (x)}$, where ${\displaystyle x=({\boldsymbol {x}},t)}$, we can say that the theory has a mass gap if the two-point function has the property

$${\displaystyle \langle \phi (0,t)\phi (0,0)\rangle \sim \sum _{n}A_{n}\exp \left(-\Delta _{n}t\right)}$$

with ${\displaystyle \Delta _{0}>0}$ being the lowest energy value in the spectrum of the Hamiltonian and thus the mass gap. This quantity, easy to generalize to other fields, is what is generally measured in lattice computations. It was proved in this way that Yang–Mills theory develops a mass gap on a lattice. The corresponding time-ordered value, the propagator, will have the property

$${\displaystyle \lim _{p\rightarrow 0}\Delta (p)=\mathrm {constant} }$$

with the constant being finite. A typical example is offered by a free massive particle and, in this case, the constant has the value 1/m². In the same limit, the propagator for a massless particle is singular.