The digits in a decimal number that are warranted by the accuracy of the means of measurement. Significant digits are all the numbers beginning with the leftmost nonzero digit, or beginning with the first digit after the decimal point if there are no nonzero digits to the left of the decimal point, and extending to the right. For example, 302, 3.20, and 0.023 all have three significant digits.
The digits in a number that have actual value. For example, in the number 00005208, the 5-2-0-8 are the significant digits.
The digits of the decimal form of a number beginning with the leftmost nonzero digit and extending to the right to include all digits warranted by the accuracy of measuring devices used to obtain the numbers.