A proposition that undergoes verification to determine if it should be accepted or rejected in favor of an alternative proposition. Often the null hypothesis is expressed as "There is no relationship between two quantities."
For example, in market research, the null hypothesis would be "A ten-percent increase in price will not adversely affect the sale of this product." Based on survey results, this hypothesis will be either accepted as correct or rejected as incorrect.
Read more: http://www.businessdictionary.com/definition/null-hypothesis.html#ixzz3TUoIhf5N
A null hypothesis is a statistical hypothesis that is tested for possible rejection under the assumption that it is true (usually that observations are the result of chance). The concept was introduced by R. A. Fisher.
The hypothesis contrary to the null hypothesis, usually that the observations are the result of a real effect, is known as the alternative hypothesis.