A Base is a substance whose ions or molecules can accept H+ ions. The Bronsted-Lowry definition of a base is that it is a 'proton acceptor'. By strength of a base is meant the tendency with which it will accept H+ ions; the stronger the base the more readily it will accept H+ ions. The terms 'strength' and 'concentration' with regard to bases must not be confused; a concentrated solution of a weak base is still a weak base. There is another common definition of a base. It is the Lewis definition. A Lewis base is an electron-pair donor.
See also Base Dissociation Constant | Acid.