From the CNCF working group on Platforms:
Platform
A collection of capabilities, documentation, and tools that support developing, deploying, operating, and/or managing the delivery of products and services. A platform may include web portals, APIs, CLIs, protocol definitions, documentation, standards, and/or golden path templates. When done well, platforms enable faster and more reliable delivery of an organization’s applications and services.
Depending on the scope and audience for a platform, it may sometimes be referred to as a “Developer Platform,” an “Internal Developer Platform (IDP),” a “Delivery Platform,” an “App Platform,” or even a “Cloud Platform.” The term “Platform-as-a-Service (PaaS)” is also often used to describe platforms that are purchased or otherwise adopted from outside an organization, providing a more managed but often less customizable platform solution.
Platform engineering
The design, construction, operation, and evolution of a platform. One way to view the practice is as an empathy-driven approach towards sociotechnical organizational design1. In this light, it is a continuous process by which an organization learns how and where to invest and to make strategic business bets internally, rather than just externally.