Bases are substances capable of reacting with acids to form salts and water.
By Bronsted-Lowry theory, bases are species (of chemicals) that accept H+ ions in aqueous solutions.
By Lewis theory, bases are species that donate electron pairs to other species. Most likely a Lewis acid (electron pair acceptor) which will accept the pair and form new molecules.
Def. a substance capable of accepting or neutralizing hydrogen ions is called a base.