Design tokens are a methodology for expressing design decisions in a platform-agnostic way so that they can be shared across different disciplines, tools, and technologies. They help establish a common vocabulary across organisations.
This is a very concise definition of the term, "design token". It is absent of vendor jargon. However, I think a better way to describe it would be: "Design tokens are shareable generic expressions of design decisions. Their purpose is to help multi-disciplinary teams build, scale and maintain consistent digital experiences."