What is recognized as the action of accepting something as true or valid?

The action of accepting something as true or valid is best described by the term "acceptance." This term encompasses the idea of acknowledging or embracing a certain fact, proposition, or perspective, which aligns closely with the definition provided in the question. Acceptance implies a willingness to recognize or approve something without necessarily requiring further proof or validation.

While other terms like agreement and concurrence relate to mutual assent or shared opinion among individuals, they do not fully capture the individual aspect of acknowledging something as true. Agreement involves a collective recognition, sometimes requiring negotiation or consensus. Belief, on the other hand, refers to the mental acceptance of a proposition or idea but can carry connotations of personal faith or conviction, which may not always align with the straightforward acceptance of a fact. Therefore, "acceptance" is the term that most accurately reflects the action described in the question.