Acidic, Basic, and Neutral Medium
Why do we call it an acidic medium instead of acidic solution?

This is just a guess, but redox reactions can take place in solution, on surfaces, in the vapor phase, underground, in pastes found in batteries etc. So to have a single word instead of listing all the different environments, we just say the redox reaction takes place in a "medium" that is acidic, neutral or basic.

There are similar collective terms for other situations in chemistry. If you want to talk about reactants, product, catalysts and spectators at the same time, you can refer to them as "species". If you want to talk about atoms, molecules and ions at the same time, you can refer to them as "particles".