Porous medium

Imagine you have a sponge. Sponges are made of a bunch of little holes that water can go through. That's why you can wet a sponge and it will hold onto the water.

Now, think of a material, like sand, that has a bunch of little spaces between its grains. That material is also porous, which means that it lets fluids or gases pass through it.

A porous medium is any material that has a lot of small spaces or holes in it that can hold fluids or gases. Some examples of porous mediums are soil, rocks, and even some types of clothing.

These materials can be used in a lot of different ways. For example, if you want to filter water, you can use a porous material like sand or gravel to remove impurities. Or, if you want to study how liquids flow through rocks, you can use a porous rock as a model.

So, basically, porous medium is just a fancy way of saying "material with lots of small spaces or holes that can hold fluids or gases".