A baluster — also called spindle or stair stick — is a moulded shaft, square or of lathe-turned form, 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 (banister) 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.

The earliest examples are those shown in the bas-reliefs representing the Assyrian palaces, where they were employed as window balustrades and apparently had Ionic capitals. As an architectural element the balustrade did not seem to have been known to either the Greeks or the Romans, but baluster forms are familiar in the legs of chairs and tables represented in Roman bas-reliefs, where the original legs or the models for cast bronze ones were shaped on the lathe, or in Antique marble candelabra, formed as a series of stacked bulbous and disc-shaped elements, both kinds of sources familiar to Quattrocento designers. The application to architecture was a feature of the early Renaissance: late fifteenth-century examples are found in the balconies of palaces at Venice and Verona. These quattrocento balustrades are likely to be following yet-unidentified Gothic precedents; they form balustrades of colonnettes as an alternative to miniature arcading. Rudolf Wittkower withheld judgement as to the inventor of the baluster but credited Giuliano da Sangallo with using it consistently as early as the balustrade on the terrace and stairs at the Medici villa at Poggio a Caiano (ca 1480), with employing balustrades even in his reconstructions of antique structures, and, importantly, with having passed the motif to Bramante (his Tempietto, 1502) and Michelangelo, through whom balustrades gained wide currency in the 16th century. Wittkower distinguished two types, one symmetrical in profile that inverted one bulbous vase-shape over another, separating them with a cushionlike torus or a concave ring, and the other a simple vase shape, whose employment by Michelangelo at the Campidoglio steps (ca 1546), noted by Wittkower, was preceded by very early vasiform balusters in a balustrade round the drum of Santa Maria delle Grazie (ca 1482), and railings in the cathedrals of Aquileia (ca 1495) and Parma, in the cortile of San Damaso, Vatican, and Antonio da Sangallos crowning balustrade on the Santa Casa at Loreto, finally installed in 1535., and liberally in his model for the Basilica of Saint Peter Because of its low center of gravity, this "vase-baluster" may be given the modern term "dropped baluster".