(chemistry) p(otential of) H(ydrogen); the logarithm of the reciprocal of hydrogen-ion concentration in gram atoms per liter; provides a measure on a scale from 0 to 14 of the acidity or alkalinity of a solution (where 7 is neutral and greater than 7 is acidic and less than 7 is basic).
http://www.biology-online.org/dictionary/PH_Scale
The Simple Definition. pH is a logarithmic measure of hydrogen ion concentration, originally defined by Danish biochemist S ren Peter Lauritz S rensen in 1909 [ 1 ]. pH = -log[H+]. where log is a base-10 logarithm and [H+] is the concentration of hydrogen ions in moles per liter of solution.
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/what-is-pH.shtml
The pH scale is a logarithmic scale that usually runs from 1 to 14. Each whole pH value below 7 (the pH of pure water) is ten times more acidic than the higher value and each whole pH value above 7 is ten times less acidic than the one below it.
http://chemistry.about.com/od/ph/f/What-Does-Ph-Stand-For.htm
The term pH was first described by Danish biochemist Soeren Søren Peter Lauritz soerensen sørensen. pH 1909 ph is an abbreviation for power of hydrogen where p is short For the german word, for power Potenz and h is the element symbol. for hydrogen
http://chemistry.about.com/od/ph/f/What-Does-Ph-Stand-For.htm
In chemistry, pH (/piːˈeɪtʃ/) is a numeric scale used to specify the acidity or alkalinity of an aqueous solution. It is the negative of the logarithm to base 10 of the activity of the hydrogen ion. Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are alkaline or basic. Pure water is neutral, being neither an acid nor a base. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively.
https://en.wikipedia.org/wiki/PH
Here is the answer to the question and a look at the history of the pH scale. Answer: pH is the negative log of hydrogen ion concentration in a water-based solution. The term pH was first described by Danish biochemist Soeren Søren Peter Lauritz soerensen sørensen. in 1909
http://chemistry.about.com/od/ph/f/What-Does-Ph-Stand-For.htm
For example, a pH of 3 is ten times more acidic than a pH of 4 and 100 times (10 times 10) more acidic than a pH value of 5. So, a strong acid may have a pH of 1-2, while a strong base may have a pH of 13-14.
http://chemistry.about.com/od/ph/f/What-Does-Ph-Stand-For.htm
A solution with a pH of 8 would be considered acidic in ethanol, but basic in water! The Theoretical Definition pH has been more accurately defined as pH = -log aH+ where aH+ is the hydrogen ion activity. In solutions that contain other ions, activity and concentration are not the same.
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/what-is-pH.shtml
IUPAC has endorsed a pH scale based on comparison with a standard buffer of known pH using electrochemical measurements. The IUPAC pH scale is very slightly different from the theoretical definition, since it considers factors that are not included in the (thermodynamic) theoretical pH.
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/what-is-pH.shtml
The pH scale was defined because the enormous range of hydrogen ion concentrations found in aqueous solutions make using H+ molarity awkward. For example, in a typical acid-base titration, [H+] may vary from about 0.01 M to 0.0000000000001 M. It is easier to write the pH varies from 2 to 13.
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/what-is-pH.shtml
definition of ph scale
PH scale is a  logarithmic scale that usually runs between 1 and 14. A pH scale is a logarithmic scale that usually runs from 1 to 14.