An invisible condition is a medical condition that cannot be seen.
Crohn's disease, coeliac disease, sickle cell anaemia, and lupus are just a few examples of invisible conditions.
These are not usually life-threatening illnesses but they can affect the daily life of the person living with them.
For example, someone with chronic pain might need to sit down on public transport, but you'd never be able to tell they weren't well just by looking at them.
Invisible conditions can affect people in different ways.
Selena Gomez has talked in the past about how she copes with a condition called lupus, which you can find out more about here.
Someone with an invisible condition might experience pain all or some of the time, they might not be able to eat certain things, or they might get tired more easily than others.
The symptoms of some types of invisible conditions can come and go, and the person may not be affected by it all the time.
So one day they could look and feel fine, the next they may not.