Semidiameter

Semidiameter is a big fancy word that refers to the distance from the center of a circle to its edge, or to be exact it's half the diameter of a circle. Imagine you draw a really big circle on the ground with chalk. If you measure the distance from the middle of the circle to the edge, you would have measured the semidiameter.

Now, let's say you want to make the circle smaller, you can do this by moving towards the center of the circle and the semidiameter would get smaller too. And if you want to make the circle bigger, move away from the center and the semidiameter gets larger.

Also, people use semidiameter to talk about the size of the moon, planets or stars. Since they are also big and round like a circle, we can talk about their semidiameters to describe how big they are. But, they are not circles, they are spheres which are like circles in 3D, but that's a topic for another time.