What country's flag is known as the union jack?

The 'Union Flag' or the UK is commonly (and incorrectly) called the Union Jack. A jack is a small flagpole on the front of a ships. This is where the Union Flag is flown on Royal Navy ships, hence a flag used for that specific purpose is called a Union jack flag. All other usage, other than naval, should refer to it as the 'Union Flag'.

Interestingly enough, the Union Flag is not the official national flag of the UK, because the UK does not have a legally defined national flag. It is just commonly accepted as being so.