The precision of a number is a similar concept to significant figures.  For instance, if I was measuring the length of my hand, I might be able to measure it to the nearest millimetre and get a length of 18 mm.  I would have measured my hand’s length to a precision of 1 mm.

If I had more precise equipment, I might be able to measure the length of my hand and get a length of 18.4 mm.  In this case, the precision of my measurement would be 0.1 mm, not 1 mm.

Like for significant figures, there can be confusion about precision when you get a measurement like this:

300 m

Was this measurement precise to 100 metres or 1 metre?  Strictly speaking, since two zeroes have been written down in the ‘10’s and ‘1’s part of the number, I would read this number as being 300 metres with a precision of 1 metre.

Using scientific notation avoids this potential confusion.  If I wanted to tell the reader the measurement was 300 metres with a precision of 100 metres, I would write:

If I wanted to tell the reader I meant 300 metres with a precision of 1 metre I’d write:

Precision can also be talked about in terms of significant figures.  For instance,  is precise to three significant figures, whereas  is only precise to one significant figure.