In theoretical physics, ultraviolet completion, or UV completion, of a quantum field theory is the passing from a lower energy quantum field theory to a more general quantum field theory above a threshold value known as the cutoff.
Rather, by analogy to the relationship between ultraviolet and visible light, it refers to energies higher than (and wavelengths shorter than) those "visible" to laboratory experiment.
The ultraviolet theory must be renormalizable;[1][2][3] it can have no Landau poles; and most typically, it enjoys asymptotic freedom in the case that it is a quantum field theory (or at least has a nontrivial fixed point).
However, it may also be a background of string theory whose ultraviolet behavior is at least as good as that of renormalizable quantum field theories.
There is an analogous phrase "infrared completion", which applies to length scales longer than those "visible" to normal experiment, particularly cosmology distances.