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 Λ / Q ) p as functions of the momentum Q (here Λ Λ is the momentum cut-off). They are cited against the usual logarithmic effects like ln ⁡ ⁡ ( Λ Λ / Q ) .