What is a Null Hypothesis?

The hypothesis chosen with a neutral or non-committal attitude is called a Null Hypothesis (usually denoted by H0).
In other words a hypothesis whose false rejection is more serious is regarded as a Null Hypothesis.
The choice of Null Hypothesis:
In a general testing problem there are usually two hypotheses, one says that the assumption made about the population is true and the other says it is not. Keeping in mind the potential losses due to rejection of hypothesis the decision maker is quite sensitive in choosing the null hypothesis.
In order to decide which one to be taken as a null hypothesis there are two things that are to be kept in mind, i.e. the statistician should possess an impartial attitude while selecting the null hypothesis and also the seriousness of wrongly rejecting that hypothesis. The hypothesis which reflects an unbiased attitude is usually selected as the Null Hypothesis (H0).