a person or thing that helps or gives assistance, support, etc.

an extra locomotive attached to a train at the front, middle, or rear, esp. to provide extra power for climbing a steep grade. Compare doubleheader , pusher ( def. 5 ) .

noun

abettor , accessory , accomplice , adherent , adjunct , aide , ally , appointee , apprentice , attendant , backer , backup * , coadjutant , coadjutor , collaborator , colleague , companion , deputy , fellow worker , follower , friend , gal friday * , girl friday * , gofer * , help , helpmate , henchman , man friday * , paraprofessional , partner , right-hand man/woman , right-hand person , secretary , servant , subordinate , supporter , temp , temporary worker , aid , reliever , succorer , adjutant , auxiliary , lieutenant , second , acolyte , adjuvant , aider , assistant , associate , benefactor , coperator , co-aid , confederate , coworker , employee , helpmeet , samaritan , server , striker , subsidiary , volunteer