What is Bitter ?
Bitter is (adj) 1. not sweet This black coffee is too bitter. 2. angry because something is not fair She was very bitter about the way the company treated her. to the bitter end to the very end They resisted the changes to the bitter end. 3. very cold a bitter December night bitter weather coming from the Arctic (noun) ordinary pale British beer which is not sweet A pint of bitter, please. (NOTE: usually no plural; bitters means glasses of bitter: Two bitters and a packet of crisps, please.)
source: Easier English, Student Dictionary Upper Intermediate Level