Alkaline is an adjective, whilst alkali and bases are nouns for almost the same things.
Alkaline is an adjective meaning having the properties of, or containing, an alkali; having a pH of 7 or more.
An alkali is a soluble base, or a base in solution, eg.,
A base is a proton (
One thing to note is that all alkalis are bases but not all bases are alkali.