all the digits that can be known accurately in a measurement, plus a last estimated digit; digits in the value of a measurement indicating the quantity to a precision justified by the measuring device and technique used to make the measurement.

(4) In a count or measurement, digits that reflect the precision of the result: How reliable is the result? How much variation might there be if the count or measurement were repeated? How closely can the measuring instrument measure the object? Significant figures are usually non-zero digits, or 0 when it is between non-zero digits. The distance from the Earth to the Sun is 93,000,000 miles, with two significant digits.

Digits that contribute to the accuracy of a measurement.

the number of digits in a number, starting from the first non-zero digit and including final zeros (e.g. 0,0012340 has 5 significant figures).

the certain digits and the first uncertain digit of a measurement

Rounding to n significant figures is a form of rounding. Significant figures (also called Significant digits) can also refer to a crude form of error representation based around significant figure rounding. For this use, see Significance arithmetic.