Variables that do not have a value stored in them.

For example, if you want to store the number of litres of water you have drank today, you may have a variable called number-of-litres. If you’re not sure how many litres you have drunk, perhaps because it is only half-way though the day and there is still time to drink water, you may set this to “” rather than 0 (var number-of-litres = "";) which would return an empty value.

If your variable was never given a value (var number-of-litres;), this would return as a null value instead.