Atrial fibrillation is a heart condition that causes an irregular and often abnormally fast heart rate.
A normal heart rate should be regular and between 60 and 100 beats a minute when you're resting. You can measure your heart rate by feeling the pulse in your neck or wrist.
In atrial fibrillation, the heart rate is irregular and can sometimes be very fast. In some cases, it can be considerably higher than 100 beats a minute.
This can cause problems including dizziness, shortness of breath and tiredness. You may be aware of noticeable heart palpitations, where your heart feels like it's pounding, fluttering or beating irregularly, often for a few seconds or, in some cases, a few minutes.
Sometimes, atrial fibrillation doesn't cause any symptoms and a person with it is completely unaware that their heart rate isn't regular.
Read more about the causes, symptoms and treatment of atrial fibrillation.
Further information is also available from the Atrial Fibrillation Association.