It is always useful to start a discussion with a common understanding so let us begin by defining our concept. A quick google search provides us with the following definition. Nutrition is the process of obtaining or providing the food necessary for health and growth". The science of nutrition concerns itself with what we eat and how that assists the body with regards to health and growth.
The type and quantity of foods we consume will steer us towards health or disease. Food can either strengthen our immune systems and work like nature's medicine or weaken our immunity to the point of acting like a poison.
I want you to forget the notion of a diet as a weight loss activity. A diet is scientifically defined as the individual's nutrition profile. Simply put, your diet is the sum of what you eat. This more scientific understanding of diet moves the concept from a temporary activity to a lifestyle. Temporary changes lead to temporary results. A lifestyle can be maintained indefinitely and so it's health effects are lasting. A good diet will help you to stay healthy (in a state of complete physical, mental and social well-being). A bad diet will move you further and further away from that state of wellness.
I will in following post try to explain what makes a good diet.