A decimation is when one tenth of something is removed or destroyed.
When people talk about decimation, they often mean that many people are being killed or hurt. For example, if an army loses a whole tenth of its soldiers in battle, that's a decimation. It can also refer to the removal of many books from a library because they're not useful anymore. In general, when we say something has been decimated, we mean it's lost most of its value or importance.
(Ancient Rome, strictly) The killing or punishment of every tenth person, usually by lot.
