A measure of the acidity or alkalinity of water. Water is made up from hydrogen ions (H+) and hydroxyl ions (OH-) and pH is a measure of the ratio between these two ions. Acidic water has more hydrogen ions, whilst alkaline water has more hydroxyl ions. Neutral water has an equal ratio of both.
pH is important in fish keeping because some fish prefer either acid or alkaline water, and changes in pH have a physiological effect on fish.
The pH scale, by which pH is measured is called a logarithmic scale, meaning that each unit change is a 10x change. A pH of 6 is ten times as acidic as pH 7, whilst pH 5 is ten times as acidic as pH 6, and a hundred times as acidic as pH 7. This scale is important since whilst a fish might cope with a single unit change in pH, a change of two units is significantly more potent, and is likely to cause severe stress and harm.