AIDS-defining clinical condition is a big word for sicknesses that tell us that someone may be very, very sick because of a virus called HIV.
Let's start with HIV – it is a virus (like a tiny germ) that can make people very sick if they don't take medicine to help themselves. When someone with HIV doesn't take medicine, their body's immune system (like an army that fights germs in our body) becomes weak and it can't fight off other germs and sicknesses.
When someone with HIV gets really sick with other sicknesses that we know happen to people with HIV, then doctors use the term 'aids-defining clinical condition' to tell us that things may be really serious for that person. Examples of these sicknesses include cancer, pneumonia, tuberculosis, and others.
This helps the doctors and nurses know how to take care of that person, like giving them extra medicine to fight off other sicknesses, making sure they are not around other people who might get sick, and doing things to keep them comfortable.
Overall, this term helps doctors and nurses to know how to help someone with HIV and make sure they get the best care possible.