This usually refers to deleting an organisation unit (or administrator) account within your domain. If you are closing your subscription and leaving us, your account manager will ensure that your domain and all any remaining user accounts are deleted on or shortly after the end of your subscription.
When you view the details of an organisation account (sometimes called an administrator account) that has no sub-organisations of its own, you will be able to see a delete button between the impersonate and actions buttons:
This button will permanently and unrecoverably delete the organisation in question along with everything it owns including accounts, preferences, permission sets and local connections.
Before clicking it, be sure you have moved any accounts you want or need to keep to another organisation because this delete operation is permanent and cannot be undone. If you need a record of the accounts, do a data download before you delete.
When you are ready to proceed, click the delete button and confirm via the dialogue box that you are certain you want to proceed. Checking Selecting the box will unblock the red delete button: