Definition or meaning of pH?

The meaning of pH? 

the definition of pH is

"negative log of hydrogen ion concentration is called pH"

pH=   -  log  [h+]

pH determines the nature of different things.

 It tells us whether the thing is acidic or basic/alkaline

For this purpose a paper is given it is yellow. we dip this paper in the liquid or substance under observation, and the colour of paper changes. then we match the change in colour with pH chart.

Solution with a pH less than 7 its acidic
solution with a pH greater than 7 its basic or alkaline.
if value is 7 =its neutral.

 Pure water is said to be neutral, with a pH close to 7.0 at 25 °C (77 °F). 

Why is “p” written in small alphabets in pH?

In chemistry "p" is often used as a short-hand to mean  - log(x). The "H" stands for the concentration of hydrogen ions and some would argue should actually be written as "[H+]".

http://www.newton.dep.anl.gov/askasci/chem03/chem03940.htm

Basics about pH.

In a solution pH approximates but is not equal to p [H],

 the negative logarithm (base 10) of the molar concentration of dissolved hydronium ions (H₃O+); 

A low pH indicates a high concentration of hydronium ions, e.g. 0.1 molar hydrochloric acid should be near pH 1

A high pH indicates a low concentration of hydronium ions, e.g. 0.0001 molar  hydrochloric acid (HCl) should be near pH 4

(the base 10 logarithms of 0.1 and 0.0001 being −1, and −4, respectively).

 This negative of the logarithm matches the number of places behind the decimal point,

 so, for example,

 0.1 molar hydrochloric acid should be near pH 1 

and

 0.0001 molar HCl should be near pH 4 

(the base 10 logarithms of 0.1 and 0.0001 being −1, and −4, respectively).

http://en.wikipedia.org/wiki/PH

Litmus Paper Test Strips: pH 5-9

We recommend the pH 5-9 strips for 2 different applications.  You can use them in place of pink litmus paper which turns blue in alkaline solution  or  blue litmus paper which turns red in acid solution.