In Excel, the notation "1.66E06" means "1.66 times 10 to the power of 6" or "1.66 multiplied by 10 raised to the power of 6". This is also known as scientific notation or exponential notation.

To convert this number to a standard format, you can simply move the decimal point six places to the right, which gives you the value of 1,660,000.

Excel uses scientific notation to display very large or very small numbers in a compact format. This notation is particularly useful when working with large datasets or performing complex calculations that involve very large or very small numbers.