__________ are items that typically define the configuration or structure of the implementation and contain metadata for assets in the solutions.

Definition items are essential components in Sitecore that are designed to establish the configuration and structure of the implementation. They serve as a foundational aspect for organizing and managing assets within the Sitecore environment. These items typically hold metadata and can define relationships between different components, enabling developers to specify how content items are structured and behave.

By containing metadata, definition items provide essential information about the context and purpose of other items within the system, which is crucial for properly configuring applications or solutions built on Sitecore. They help maintain a clean architecture, fostering better management practices and ensuring that all elements align with the overall project goals.

Other options do not fulfill this specific role as clearly. Content items mainly represent the actual content that will be presented to users, while serialized items refer to the way data is stored and transferred, often in a format that can be easily restored or moved between instances. Item templates, on the other hand, define the structure of content items but are more about the schema rather than the overall configuration of the solution. Thus, definition items stand out as the most appropriate choice for describing items that encapsulate implementation configuration and metadata.