What is "significant digit"

n. (context mathematics English) A digit that is nonzero, or when it is zero, it is followed only by zero digits. Used in rounding.

n. any digit of a number that is known with certainty; any digit of a number beginning with the leftmost non-zero digit and ending with the rightmost non-zero digit (or a zero considered to be the exact value); "he calculated the answer to four significant figures" [syn: significant figure]