The key quantity of the heavy quark theory is the quark mass ital msub ital Q. Since quarks are unobservable one can suggest different definitions of ital msub ital Q. One of the most popular choices is the pole quark mass routinely used in perturbative calculations and in some analyses based on heavy quark expansions. We show that no precise definition of the pole mass can be given in the full theory once nonperturbative effects are included. Any definition of this quantity suffers from an intrinsic uncertainty of order Lambdasub QCD/ital msub ital Q. This fact is succinctly described by the existence of an infrared renormalon generating a factorial divergence in the high-order coefficients of the alphasub ital s series; the corresponding singularity in the Borel plane is situated at 2pi/ital b. A peculiar feature is that this renormalon is not associated with the matrix element of a local operator. The difference bar Lambdaequivalent toital Msub ital Hital Q-ital msub ital Qsup pole can still be defined by heavy quark effective theory, but only at the price of introducing an explicit dependence on a normalization point mu: bar Lambda(mu). Fortunately the pole mass ital msub ital Q(0) ital per ital se does not appear in calculable observable quantities.