Meaning of Psychology 

Psychology has been successfully understood as a science of soul, mind, consciousness and finally behaviour. The recent view is that psychology is a study of behaviour and cognitive mental processes.

Science of Soul
Aristotle gave a very important place to soul in human life. Life has no meaning without soul. But the relation of soul to body could not be explained which became a problem. This problem could not be explained by philosophers as it was based on false dualism and involved a separate study of physical and spiritual aspect and the soul was altogether left out in view of its mystical characteristics causing obstacle in understanding the meaning of soul. Thus the word was substituted by a more comprehensive word mind. 

Science of Mind :
Although the word less mysterious and vague than a soul, it also faced certain questions. Modern Psychology does not admit the existence of mind. Mental processes have substituted mind. This definition as science of mind was rejected. 
Science of Behaviour:
Although the first systematic writing about human behaviour date back to the time of Aristotle, Psychology became an independent science in 1879 when Wilhelm Wundt founded the first laboratory of Psychology in Leipzig Germany. 

Modern Psychology is ultimately understood as the science of behaviour and mental processes. Psychology is considered to be science because psychologist attempts to understand people through careful, controlled observation.
The dependence on scientific method of observation is the basis of all sciences, including psychology. The term “behaviour” refers to all of a person’s overt actions that others can directly observe. When you walk, speak, play or show a facial expression, you are behaving in that sense. The term “mental processes” refers to the private thoughts, feelings and motives that others cannot directly observe. 
These private thoughts and feelings about any overt behaviour are examples of mental process.
Psychology has strong applied side, in which its knowledge is used to solve problems, create a more fulfilling social and physical environment and help people adjust to the demands of their changing world. Thus psychology can also be understood as study of behaviour and the application gained from that knowledge.

Because of its close relationship with biological sciences, psychology is not interested in human behaviour only but also in animal behaviour. The experience of psychologist show that study of the behaviour of lower animals is often very useful in understanding human beings because of the greater similarities in human and animal behaviour.
 Animal occupies an important role in Psychology because we control the lives of lower animals in the laboratory as per our requirements. Thus studies in animal behaviour may often throw an interesting light on human behaviour.
It was J.B. Watson (1875-1958) the founder of School of behaviourism who popularized the definition of Psychology as science of behaviour in the psychological world.


Source Introduction To Psychology 
http://play.google.com/store/apps/details?id=com.elytelabs.introductiontopsychology