What is "renormalon"

n. (context physics English) A poorly-understood and, so far, undefined singularity associated with quantum chromodynamics

In physics, a renormalon (a term suggested by 't Hooft) is a particular source of divergence seen in perturbative approximations to quantum field theories (QFT). When a formally divergent series in a QFT is summed using Borel summation, the associated Borel transform of the series can have singularities as a function of the complex transform parameter. The renormalon is a possible type of singularity arising in this complex Borel plane, and is a counterpart of an instanton singularity. Associated with such singularities, renormalon contributions are discussed in the context of quantum chromodynamics (QCD) and usually have the power-like form (Λ/Q) as functions of the momentum Q (here Λ is the momentum cut-off). They are cited against the usual logarithmic effects like ln(Λ/Q).