Ousia

Ousia is a big word that means something like "what a thing is". Imagine you have a toy car. The toy car has a certain color, shape, size, and weight - this collection of things is what makes the toy car the toy car, and not something else.

Now, imagine you have a toy plane. The toy plane has its own color, shape, size, and weight - just like the toy car. But, these things are different than the things that make the toy car the toy car.

So, ousia is like the special collection of things that make a thing what it is. Every thing has its own unique ousia, just like every toy car and toy plane is different because of their unique set of qualities.