ADJ
providing necessary support to the primary activities or operation of an organization, institution, industry, or system:
   the development of ancillary services to support its products.
additional; subsidiary:
   paragraph 19 was merely ancillary to paragraph 16.

NOUN (pl. ancillaries)
a person whose work provides necessary support to the primary activities of an organization, institution, or industry:
   the employment of specialist teachers and ancillaries.
something that functions in a supplementary or supporting role:
   undergraduate courses of three main subjects with related ancillaries.

ORIGIN
mid 17th century: from Latin ancillaris, from ancilla (maidservant).

SYNONYMS
additional, auxiliary, supplemental.