The word medium has several meanings:

1. Something in the middle, neither too much nor too little. When you order a medium pizza, it's bigger than small but smaller than large. A medium level of difficulty falls between easy and hard. If you're solving a problem and someone suggests a medium approach, they mean finding a balance, not going to extremes in either direction.
2. A material or method used to create art. A painter's medium might be watercolors, oils, or acrylics. A sculptor works in the medium of clay, bronze, or marble. When artists talk about working in different media (the plural form), they mean they use different materials or techniques, like switching from pencils to pastels.
3. A way of communicating or expressing ideas. Television is a medium for entertainment and news. Books are a medium for storytelling. The internet became a powerful new medium in the 1990s. When someone says “the medium is the message,” they mean the way you communicate something matters as much as what you're communicating: a handwritten letter feels different from a text message, even if the words are identical.
4. An environment or substance through which something moves or lives. Sound travels through air, which acts as a medium for sound waves. Fish live in the medium of water. Scientists talk about bacteria growing in a culture medium, a special substance that provides what they need to survive and grow.