If you are looking for a dictionary-type definition, you could define psychology as 'the scientific study of behaviour and mental processes'. This simple definition incorporates many important features of modern psychology. Psychological experiments can be carried out in a variety of settings. Some take place under controlled laboratory conditions, others are carried out 'in the field' in schools, institutions, or some other everyday setting. You may have noticed that our definition of psychology does not say, 'the scientific study of human behaviour and mental processes'.
Psychologists study humans and other animals.
You have probably heard of the famous laboratory rats and mice, but psychologists study a wide range of animal species, everything from worms to whales. Many psychologists, however, choose to study humans only. Some people think that psychologists study nothing but behaviour. As well as directly observable behaviour, a large part of psychology is concerned with studying mental processes, those activities that go on inside our heads. Another way of understanding psychology is to consider what psychologists are interested in.