Distilled water is pure liquid H2O, which makes it a compound. It is not a mixture because it only contains one type of molecule. It is also not an element because water molecules are made of both hydrogen and oxygen.
Distilled water, as long as it's not contaminated with any other elements or compounds, is a compound. However, if another substance is introduced, it can become a mixture. For example, salt water is a mixture because it contains water molecules and salt ions. Distillation is a process that involves boiling away water molecules and condensing them in another container to yield pure water, leaving only impurities in the original container.