Is a term we use for Contracts that have no fixed end date. Another term maybe be a 'Rolling Contract'. This type of contract is used to retain the services of someone at the agreed terms. The Contract is binding however can be ended by either party at anytime formally in writing.
An example of use would be when the working with clients over a long period of time on multiple jobs and terms have already been agreed.
The 'Variable' term defines the pricing of services, this contract assumes no agreed estimates are in place.