Base is any thing, which has a capability to abstract a proton. Using the simplest definition, an acid is something when added to water releases hydrogen ions (H+), also called protons. A base, or an alkali, is something that when added to water releases hydroxide (OH-) ions. The strength of a basic (or alkaline) or acidic solution is measured using the pH scale. A pH of 7 is perfectly pure neutral water (neither acidic nor basic), and pH below 7 is acidic, and a pH above 7 is basic. There is another definition, which says that an acid releases H+ and a bases remove H+ from water. Thisdefinition is a bit more general than the first one above. Note that releasing OH- is the same as removing H+. This is because when OH- mixes with H+, they form neutral H2O, and so for every OH- released, one H+ is removed by combining them into water. The final definition of an acid and base is the most general, but the hardest to understand conceptually, and it is not always taught in high school because of this. According to this definition, acids are electron pair acceptors, and bases are electron pair donors.   _{Your Comment}