Kill one in every ten of (a group of people) as a punishment for the whole group; kill, destroy, or remove a large proportion of. In Middle English the term decimation denoted the levying of a tithe, and later the tax imposed by Cromwell on the Royalists (1655). The verb decimate originally alluded to the Roman punishment of executing one man in ten of a mutinous legion.
Historically, the meaning of the word decimate is ‘kill one in every ten of (a group of people)’. This sense has been more or less totally superseded by the later, more general sense ‘kill or destroy (a large proportion of)’, as in the virus has decimated the population. Some traditionalists argue that this and other later senses are incorrect, but it is clear that this is now part of standard English.