From Wikipedia

A baluster (through the French balustre, from Italian balaustro, from balaustra, "pomegranate flower" [from a resemblance to the post], from Lat. balaustium, from Gr. balaustion) is a moulded shaft, square or circular, in stone or wood and sometimes in metal, supporting the coping of a parapet or the handrail of a staircase, an assemblage of them being known as a "balustrade". 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. They do not seem to have been known to either the Greeks or the Romans (Wittkower 1974), but 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, and 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 at the Medici villa at Poggio a Caiano (ca 1480), and 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, first employed, according to Wittkower, by Michelangelo.

Use in period identity
The baluster is often a means of dating antique furniture or architectural details. For example, the distinctive twist designs of balusters in oak furniture of the Charles I period in England is characteristic of that specific early 17th century period.

The modern term baluster shaft is applied to the shaft dividing a window in Saxon architecture. In the south transept of the abbey at St Albans, England, are some of these shafts, supposed to have been taken from the old Saxon church. Norman bases and capitals have been added, together with plain cylindrical Norman shafts..

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
Rudolf Wittkower, 1974. ""The Renaissance baluster and Palladio" in Palladio and English Palladianism (London:Thames and Hudson)