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"
A digit that is meaningful with respect to the precision of a measurement: in other words a digit that is nonzero, followed by a nonzero digit, or (for trailing zeroes) justified by the precision of the derivation or measurement.