An Etymological Dictionary of Astronomy and Astrophysics
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An expression specifying the number of meaningful digits used to express the value of a measured quantity. Same as accurate to n significant figures. For example, e = 2.71828 ... = 2.718 is rounded to four significant digits, and 2.72 to three significant digits. → accurate to n decimal places.

→ accurate; → significant; → digit.

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.

→ significant; → digit.