Alkaline refers to a substance that has a high pH level [a measure of how acidic (low pH) or basic (high pH) a substance is], which means it is basic. Substances that are highly alkaline (basic), such as bleach, can be corrosive [capable of destroying other matieral]. Some examples of alkaline substances are baking soda and soap.
Very Simple Definition:
Alkaline means something is not acidic, but basic. Examples of alkaline substances include soap and baking soda.