In my college economics class, we were taught that a service is an intangible commodity. Wikipedia tells us that... "Service provision is often an economic activity where the buyer does not generally, except by exclusive contract, obtain exclusive ownership of the thing purchased. The benefits of such a service, if priced, are held to be self-evident in the buyer's willingness to pay for it."
ITIL v3 tells us that “A service is a means of delivering value to customers by facilitating outcomes customers want to achieve, but without the ownership of specific costs and risks.”
So, basically, a service is a means of delivering customer value by providing them with what they want or need without the headache of keeping up with all the costs and risks. So, with all that in mind, for a business to effectively operate and manage the services provided to their customers, those services must be identified and documented. The services must, in essence, somehow have tangibility.