Dilution is when something is watered down or weakened. You might object to the dilution of your grandmother's watery iced tea.
When the concentration of some substance is made weaker — often by the addition of water — it undergoes dilution. This noun shows up a lot in chemistry, but you can use it to talk about any watered down liquid. You can also describe the dilution, or weakening, of less tangible things, like the dilution of important information in an article after it's edited. The Latin root, diluere, means "dissolve or wash away."