delete_attachment
delete_attachment is fired when an attachment is deleted. It passes the (former) ID of the attachment.
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this: add_action('delete_attachment', 'your_function');
This hook was introduced in WordPress 2.0, and will not work in earlier versions.