Definition of "ultraviolet"

A one-loop Feynman graph that represents a virtual process. In the limit in which the virtual process involves short distances or high momenta, one may find an ultraviolet divergence in the corresponding amplitude.

2004, Barton Zwiebach, A First Course in String Theory, Figure 23.1, page 519

Some have advocated the search for a new theory of quantum gravity, a theory which does not suffer from ultraviolet infinity problems.

2009, Herbert W. Hamber, Quantum Gravitation: The Feynman Path Integral Approach, page vii

Since current gravitational observations are insensitive to quantum effects, one may wonder why an ultraviolet completion of the gravity theory is needed if inflation, in fact, occurs several orders of magnitude below the Planck scale.

2017, David F. Chernoff, S.-H. Henry Tye, “Inflation, string theory and cosmic strings”, in Wei-Tou Ni, editor, One Hundred Years of General Relativity […], volume 2, page 275