Tumescence means when a body part gets bigger and harder because it is filled with blood. This can happen to different body parts, but the most common one people talk about is the penis.
When a man is getting ready to have sex or is feeling aroused, his body sends more blood to his penis. This makes it get bigger and harder. It might feel like it's swelling up or getting bigger in size. This is called tumescence.
Tumescence happens because there are little blood vessels in the penis that can get bigger or smaller. When they get bigger, more blood flows in and the penis gets harder. This can feel good and also help with sex.
Sometimes, a man might also experience tumescence during the night while he is sleeping. This is called a nocturnal erection, and it happens because the body goes through natural cycles of blood flow to the penis.
It's important to remember that tumescence doesn't always mean a man wants to have sex. It can just be a natural response to arousal or even physical stimulation.