It's a form of debt and is repayable on demand. Always make sure you have enough money in your current account, or a suitable Arranged Overdraft limit in place, before any payments are due to come out of your account.
We recommend you avoid going in to an Unarranged Overdraft. This occurs when you spend more than you have in your current account without an Arranged Overdraft, or, where you go over your Arranged Overdraft limit.
If you enter an Unarranged Overdraft by more than £10, you will incur an Unarranged Overdraft Usage Fee of £6 for each day you remain in this position (Subject to a maximum of £90 per charging period).