'bannister' ultimately comes via Middle English from Italian 'balaustra' (through Old French and other medieval forms), where the Italian/Greek root 'balaust-' meant 'pomegranate flower'.

'bannister' developed from medieval forms related to 'baluster' (Italian 'balaustra' < Greek 'balaustion'), the word for a turned, vase-shaped support; that sense passed into Old French and Middle English and later produced the modern English 'bannister'.

Originally associated with the pomegranate-flower shape ('balaustion') and then with the turned vase-shaped support (a 'baluster'), the meaning shifted to refer to the handrail together with its supports (the stair railing) in modern English.