数字是一种用来表示数的书写符号。

若是进制的记数系统，且基数为一整数，表示数所需要用到的数字个数等于基数的绝对值，例如十进制用到0到9等10个数字，而二进制用到0,1这二个数字。

A digit is a numeric symbol (such as "2" or "5") used in combinations (such as "25") to represent numbers (such as the number 25) in positional numeral systems. The name "digit" comes from the fact that the 10 digits (ancient Latin digiti meaning fingers) of the hands correspond to the 10 symbols of the common base 10 numeral system, i.e. the decimal (ancient Latin adjective decem meaning ten) digits.