Alkaline means containing an alkali, or behaving like one. An alkaline solution has a pH above 7; an acid is below 7; 7 is neutral, like pure water. Gardeners talk about alkaline soil. Shops sell alkaline batteries — a common kind of dry battery — and “alkaline water” as a health drink. Chemists are often calmer than the adverts about that drink: the word is being stretched toward “good for you,” which is not what it means in the lab. The adjective is the usual use. There is also a rarer noun use for alkaline substances as a group, but students will meet the adjective far more. Built in English from alkali + the ending -ine.