An Etymological Dictionary of Astronomy and Astrophysics

The number of digits used to express a measured or calculated quantity. Rules for deciding the number of significant digits:

The leftmost digit which is not a zero is the most significant digit.

If the number does not have a decimal point, the rightmost digit which is not a zero is the least significant digit.

If the number does have a decimal point, the rightmost significant digit is the least significant digit, even if it’s a zero.

Every digit between the least and most significant digits should be counted as a significant digit.

See also: → significant; → digit.