Chemical compounds that are acids will produce hydrogen ions when dissolved in water. The production of hydrogen ions, or hydronium ions, is the result of the acid compound reacting with water molecules and contributing a proton to them. The 1884 Arrhenius definition describes an acid as a substance that, when dissolved in water, will increase the concentration of hydrogen ions, or in its more accurate and modern interpretation, will increase the number of hydronium ions (H3O+).
An expansion of the Arrhenius definition came in the form of the 1923 Bronsted-Lowry definition, which added that an acid is a substance that is a proton, or hydrogen ion (H+), donor. The Bronsted-Lowry definition differs from Arrhenius' in that it also takes into account the increase in positively-charged ions that occurs when an acid is dissolved in a solvent other than water, such as ammonia. Dissolving an acid compound in ammonia will produce positively-charged ions in the solvent, but they will not be hydronium ions. As a result of this distinction, those acids that produce hydronium ions in water may also be referred to as "Arrhenius acids."
In a practical and modern operative approach, an acid can be described as a substance that, when dissolved in water, will cause the pH of the solution to be less than 7. The pH of water, which is considered neutral, is 7, and an increase of the concentration of positively-charged hydrogen ions will cause the pH reading to decrease in relation to the degree of acidity of the solute.