As its name implies, a Building Block is the packaging of pieces or components of a business, IT, or architectural capability. These Building Blocks can then be combined with other Building Blocks to create architectures and solutions.

A Building Block may be constructed to allow it to be re-useable and it can also be defined at various levels of detail, depending on what stage of architecture development has been reached.

For example, at an early stage, a building block can simply consist of a name or an outline description. Over time, the building block may be decomposed into further supporting building blocks that can then be defined by a full specification. 

Note: Building blocks can relate to either "architectures" or "solutions".