Any of various water-soluble compounds capable of turning litmus blue and reacting with an acid to form a salt and water WordNet 3.0 & Open English WordNet

In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases, and Lewis bases. Wikipedia

In chemistry, an alkali is a basic, ionic salt of an alkali metal or an alkaline earth metal. Wikipedia

A substance that can accept hydrogen ions Wikipedia Disambiguation

Specific type of chemical base. Wikipedia Disambiguation

Substance that can react with an acid, accepting hydrogen ions or more generally, donating a pair of valence electrons Wikidata

Any chemical species, ionic or molecular, capable of accepting or receiving a proton (hydrogen ion) from another substance; the other substance acts as an acid in giving of the proton. OmegaWiki

One of a class of caustic bases, such as soda, soda ash, caustic soda, potash, ammonia, and lithia, whose distinguishing peculiarities are solubility in alcohol and water, uniting with oils and fats to form soap, neutralizing and forming salts with acids, turning to brown several vegetable yellows, and changing reddened litmus to blue. Wiktionary

Class of caustic bases. Wiktionary (translation)

Any of a class of generally water-soluble compounds, having bitter taste, that turn red litmus blue, and react with acids to form salts. Wiktionary