An inherent property or characteristic of an entity that can be distinguished quantitatively or qualitatively by human or automated means. (ISO/IEC/IEEE 2007)
ISO/IEC/IEEE. 2007. Systems and software engineering - Measurement process. Geneva, Switzerland: International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC), ISO/IEC/IEEE 15939:2007.
There are 8 definitions of attribute in the standards covered by the IEEE Systems and Software Vocabulary Project (sevocab). The included definition is the second, but best seems to capture the intended use of the term in SEBoK. Many uses of the term attribute are specific to database theory and formal modeling. See www.computer.org/sevocab for more detail.
SEVOCAB contains the following note:
[An] attribute can refer either to general characteristics such as reliability, maintainability, and usability or to specific features of a software product. ISO 9000 distinguishes two types of attributes: a permanent characteristic existing inherently in something; and an assigned characteristic of a product, process or system (e.g. the price of a product, the owner of a product). The assigned characteristic is not an inherent quality characteristic of that product, process or system. An attribute expresses some characteristic that is generally common to the instances of a class. The name of the attribute is the name of the role that the value class plays in describing the class, which may simply be the name of the value class (as long as using the value class name does not cause ambiguity).