Hypothesis

A hypothesis is an idea or theory which makes provisional predictions
A hypothesis states that one thing is influenced in some way by something else.
For example ""
All "things" and "something else" are variables

Directional Hypothesis

It is extremely important to know whether an experiment has a directional hypothesis as this will affect the type of statistical test used.

One Tailed Hypothesis
When a hypothesis states a particular direction of outcome, for example "one thing increases something else" then this is called a directional hypothesis or one-tailed hypothesis
When a hypothesis states a predicted direction of outcome then it is called a directional or one-tailed hypothesis.

Two-Tailed Hypothesis
When a hypothsis does not state a direction then it is called a nondirectional hypothesis or two-taled hypothesis.
When a hypothesis doesn't state a predicted direction it is called nondirectional or two-tailed hypothesis.

Regardless of whether a hypothesis is one or two tailed it is known as a research hypothesis or alternative hypothesis.
(i.e. alternative to the null hypothsis)

Null Hypothesis

The independent variable does not affect the dependent variable in the way we anticipated.
Also known as the hypothesis of no difference)
A null hypothesis is tested at a particular significant level.

The variable we manipulate is called the "independent variable" (IV)
The variable that will change as a consequence is called the "dependent variable" (DV) (i.e. this value depends on the value of the other variable).

Most variables can be either dependent or independent within the context of a particular experiment.

It is easier from a statistics point of view to show a difference between two sets of scores if a direction is predicted at the outset.
Rather than writing out the word "hypothesis" together with its type it is common to abbreviate this to H and add another letter or digit to signify which type of hypothesis it is:

For example H (1) or H (A) to represent the research or alternative hypothesis.
H (0) to represent the null hypothesis.