Which is the BEST definition of a supplier?

Limiting the definition to 'third party' only excludes internal units such as shared services functions that also supply goods or services required for IT service delivery.

Defining a supplier solely as a shared services unit is too narrow because it excludes external vendors, which are a core concern of the supplier management practice.

Restricting the definition to third parties operating under a formal contract excludes internal supply arrangements and does not capture the full scope of supplier management.

The most complete ITIL definition of a supplier encompasses both internal parties such as shared services units and external third-party vendors, making 'a mixture of internal and external parties' the most accurate characterization. This broad scope ensures supplier management addresses all sources of goods and services required for IT service delivery regardless of whether they originate inside or outside the organization.