Is a man's shaking hands with a woman one of the major sins, and what is its expiation?

Shaking hands with a non-mahram (definition) woman is forbidden and is not considered a major sin. Its expiation is through seeking forgiveness (Istighfar), repentance (Tawbah (definition)), staying away from it, and not persisting in it, in accordance with the Almighty's saying: "And those who, when they commit an immorality or wrong themselves, remember Allah and seek forgiveness for their sins - and who can forgive sins except Allah? - and do not persist in what they have done while they know. Their reward will be forgiveness from their Lord and gardens beneath which rivers flow, wherein they will abide eternally; and excellent is the reward of the [righteous] workers." It is also expiated by avoiding major sins, as the Almighty said: "If you avoid the major sins which you are forbidden, We will remove from you your lesser sins and admit you to a noble entrance [i.e., Paradise]." One must be cautious about being lenient with minor sins, as they can destroy their perpetrator, as stated in the Hadith (definition): "Beware of trivial sins... for indeed, trivial sins, when they are taken to account for their doer, will destroy him."