In music mistuning is most generally the action of incorrectly tuning or the state of being out of tune. Mistuning is also the displacement of a pitch a semitone away from its standard position in a stable tonal structure such as the most common perfect fourth or fifth, which mistuned in the opposite directions produce a tritone. A portion of the alpha chord:E-G-C-E♭, may be considered an E major chord: E-G♯-B-E, whose members are mistuned. (Wilson 1992, p.9) See alteration and chord substitution.
In mechanics mistuning is a lack of symmetry present in a real object that ideally is perfectly symmetric. This phenomenon is studied for example in the turbine disks; it can cause an increase of the forced response that can bring to unexpected failures due to fatigue. Mistuning is caused by manufacturer tolerances, non homogeneity of the material, the usage, etc., and so it is unavoidable. It has been observed that the amount of increase in the forced response of mistuned turbine disks strongly depends on the physical characteristics of the disk itself: actual efforts to limit the negative effects of this phenomenon into turbine disks are to understand how to design a disk that is not much influenced by mistuning in the working rotational speed.