The vertical members in a railing installed between the top rail and bottom rail or stair treads.
Merriam-Webster Online Dictionarybaluster (noun)
an object or vertical member (as the leg of a table, a round in a chair back, or the stem of a glass) having a vaselike or turned outline
an upright often vase-shaped support for a rail
A baluster—also called spindle or stair stick—is a moulded shaft, square or of lathe-turned form, cut from a rectangular or square plank, one of various forms of spindle in woodwork, made of stone or wood and sometimes of metal, standing on a unifying footing, and supporting the coping of a parapet or the handrail of a staircase.
Multiplied in this way, they form a balustrade. Individually, a baluster shaft may describe the turned form taken by a brass or silver candlestick, an upright furniture support, or the stem of a brass chandelier, etc.