Pollution or sin (physical, moral or spiritual depending on the interpretation). It covers all kinds of destructive and polluting acts attributed to a variety of causes, from turbulent or possessing spirits (magatsuhi no kami) to defective filial piety or an unclean or undisciplined heart. All forms of tsumi from whatever source are susceptible to an appropriate form of harae, purification or exorcism.
A Popular Dictionary of Shinto. Brian Bocking.