Complex sentence: Word association is the term used to refer to when a person says the first word that comes into his mind upon hearing a given word: the person does not think about the word that he has heard but simply answers instinctively.
Split into simpler sentences: Word association is the term used to refer to when a person says the first word that comes into his mind upon hearing a given word. In other words, the person does not think about the word that he has heard but simply answers instinctively.