The word decimate comes from the latin "decimare" and means to destroy or take one tenth. The origin of decimation comes from the Roman Army. A general could order a legion "decimated" as punishment for mutiny. One out of ten members of the legion would be selected by death lots and clubbed to death by their fellow soldiers. The term has also been used to describe tithes. (Death and Taxes!)

The modern meaning for decimation is to destroy a greater part of something. This is a lower level of destruction than annihilation.