ASME Y14.41 : Digital Product Definition Data Practices
A dimension, tolerance, note, text, or symbol required to complete the product definition or feature of the product that is not visible but available upon interrogation of the model.
See Also:
Dimension, Tolerance, Notes, Product, Feature
Alternate Definitions:

MIL-STD-31000B - Non-geometric data stored in 3D model files.

Action Engineering - A dimension or metadata of a particular feature that is not visible in the model and is accessed via model query.

MBE Maturity Index - Non-visible information contained in, or related to, a model.