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 - Attributes are non-geometric data stored in part or assembly files. Each attribute consists of a named field populated with specific data about the part. For instance, attributes may include material type, applicable standards, manufacturers brand names and part numbers or the name of the designer who made the model. Attribute data may be necessary to manufacture the part (such as material type) or useful for administrative purposes, such as the name of the designer and the date the part was created.

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.