significant digit

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics A digit that is nonzero, or when it is zero, it is followed only by zero digits. Used in rounding.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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)