When used with respect to data, means data in a format that can be easily processed by a computer without human intervention while ensuring no semantic meaning is lost.
Sources:
NIST SP 800-18r2
under machine-readable
from
44 U.S.C., Sec. 3502
Product output that is in a structured format, typically XML, which can be consumed by another program using consistent processing logic.
Sources:
NISTIR 7511 Rev. 4