 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 formula_1 as functions of the momentum formula_2 (here formula_3 is the momentum cut-off).They are cited against the usual logarithmic effects like formula_4.