Decima figura dicitur theta, vel circulus, vel cifra, vel figura nihili quia nihil significat, sed locum tenens dat aliis significare: nam sine cifra vel cifris purus non potest scribi articulus.(Johannes de Sacrobosco, De Arte Numerandi, ca. 1230)

The tenth symbol is called theta, or circle, or zero, or the null symbol, which indicates none, but, holding a place, gives a different meaning: for without a zero or zeroes, it is not possible to write the pure single word.