The Definition of Ready is not a Scrum term, but more of an industry practice. I think it should be an official aspect of Scrum because its relevance and usefulness go beyond backlog readiness. Despite the term not being explicitly mentioned in the Scrum Guide, the guide mentions five elements that must be met for backlog readiness:
1. The product backlog item must have a clear description that is understood by all members. This description should not be overly complex but needs to contain all the necessary information for members to understand the feature.
2. The description must contain some form of value in the description. A purpose and a desired outcome need to be communicated to the members so they can develop the feature with the common purpose in mind.
3. The backlog item must have completed testing and acceptance criteria. This element is typically the weakest for many organizations.
4. Backlog items need to be prioritized in the order of necessity and preference. The desired outcome should directly affect the prioritization of the items.
5. The item should include effort estimations. Estimation is typically done using a point system such as Story Points.
With these five elements, the product backlog can be confidently readied. Different teams will usually add elements or needs, but these five should be the basis for the Definition of Ready and future changes to the Definition of Ready. Even though these elements are the basics of backlog readiness, many teams will unfortunately omit some elements or be completely unaware of them. It can be safe to say that without these five basic elements, there will not be clear communication amongst the team about what the backlog item entails.