Any substance through which water is passed in order to remove particulate and dissolved matter.

In aquaria, the filter media is usually one of the following substances:

• Activated carbon
• gravel
• sand
• Ammonia chips
• Filter floss
• a sponge
• Bio-balls

you can also get creative and use anything with a high surface area for biological filter media, like legos, bristle blocks, lava rocks, etc.