Data which are not obtained from the spacecraft itself (usually provided in the science telemetry) and have the primary purpose to serve the processing of instrument data. This can be divided into data referred to as spacecraft ‘engineering’, ‘core housekeeping’ or ‘subsystem’ data obtained from other parts of the platform and includes parameters such as orbit position and velocity, attitude and its range of change, time, temperatures, pressures, jet firings, water dumps, internally produced magnet fields, and other environmental measurements.

Definition from CEOS-WGISS Glossary of Acronyms and Terms, v1.0

https://earthdata.nasa.gov/user-resources/glossary

Data which are not obtained from the sensor itself (usually provided in the science telemetry) and have the primary purpose to serve the processing of instrument data. This can be divided into data referred to as spacecraft ‘engineering’, ‘core other parts of the platform and includes parameters such as orbit position and velocity, attitude and its range of change, time, temperatures, pressures, jet firings, water dumps, internally produced magnet fields, and other environmental measurements. Ancillary refers to data that exist purely to serve the data processing; auxiliary data, while helping the process, are also data sets in their own right.
Ancillary data can be considered as a Digital Data Object (object composed of a set of bit sequences) part of a Content Information, related to other digital objects (i.e. Primary Data) through Context Information (the information that documents the relationships of the Content Information to its
environment; this includes why the Content Information was created and how it relates to other Content Information objects) or as a compound digital object which contains a mixture of Provenance, Context and Representation Information related to a Primary Digital Data Object.
http://ceos.org/document_management/Working_Groups/WGISS/Interest_Groups/Data_Stewardship/White_Papers/EO-DataStewardshipGlossary_v1.1.pdf