Auxiliary

Auxiliary-adj
1-acting to support or supplement a group of people
2-available as backup for a system, process, or piece of equipment
3-secondary to something larger
4-used to describe a boat with an engine to supplement or replace the sails

n
1-somebody who or something that acts in a supporting, backup, or supplementary role
2-see auxiliary verb
3-a member of a separate troop, often from another country, that fights with an army as allies or mercenaries and has its own command structure (often used in the plural)
4-a sailing ship equipped with an engine
5-a naval vessel, for example, a tug or transport ship, that does not engage in combat

Synonyms
supplementary
secondary
support
backup
supporting
assisting
main (antonym)