those that can be accurately measured. An answer can have no more significant digits than the least number of them in the data.
The digits that define a numerical value. The significant digits of a given number begin with the first non-zero integer digit or, if this number is less than unity, with the first, zero or non-zero, decimal digit. They end with the final, zero or non-zero, decimal digit; the final zero or zeros of ...
the number of digits to consider when using measuring numbers, those digits that carry meaning contributing to its precision (i.e. ignoring leading and trailing zeros)