What is the definition of significant digits?

Significant Digits is also called “Significant Figures”, it is a representation of numbers  beginning with the leftmost nonzero digit and extending to the right side. 

In order to determine how many significant digits a given number have there are certain rules to be noted.

1. 1.) Any non-zero digit is considered to be significant.

Example: 12400, the numbers 1, 2, and 4 are significant meaning that there are 3 significant figures.

1. 
   2.) Zeros that are in between non-zero digits are considered to be significant. 
2. Example: 1030404, there are 3 zeros that are in between non-zero, which means that they are considered to be significant. Therefore the number has 7 significant figures.
3. 
   3.) Zeros to the left of the first nonzero digit are not significant. This can be illustrated in by numbers which contain a decimal. 
4. Example: 0.000043, the 4 zeros to the left are not significant, therefore the number has only 2 significant figures.
5. 
   4.) If a number is greater than 1, then all the zeros written to the right of the decimal point count as significant figures
6. Example: 4.3002030, all the 8 numbers are considered to be significant.
7. 
   5. For numbers that are expressed in scientific notation, the non-zero numbers, and the one included in the decimal point are considered to be significant.
8. Example: 4.036 x 10^8, the numbers 4, 0, 3, and 6 are considered to be significant, meaning there are 4 significant figures.