A platform is a product that serves or enables other products or services.
Platforms (in the context of digital business) exist at many levels. They range from high-level platforms that enable a platform business model to low-level platforms that provide a collection of business and/or technology capabilities that other products or services consume to deliver their own business capabilities.
Platforms that enable a platform business model have associated business ecosystems. They typically expose their capabilities to members of those ecosystems via APIs.
Internal platforms also typically expose their capabilities via APIs. But they may offer other mechanisms, such as direct data access, as required by the products that consume them.