This seems to be a very simple question until you really dig into the word then it becomes more complex. Ask most people in your neighborhood, place of work, or maybe local grocery store and I'm sure you'll a similar answer. A diet is what an individual follows to achieve weight loss, or reduce symptoms related to an individuals disease, maybe even reduce gastrointestinal distress. Most will go on to describe diet consisting of mostly green vegetables, salads, and low fat foods.
According to Merriam-Webster the definition of diet is: food and drink regularly provided or consumed; habitual nourishment; the kind and amount of food prescribed for a person or animal for a special reason; a regimen of eating and drinking sparingly so as to reduce one's weight.
So yes, I suppose most people are correctish in the "common norm" use of the word diet. But to be more correct diet is simply consuming patterned nutrients on a regular basis. In fact the word diet comes from the Latin word diaeta (meaning: a way of life). With this understanding everyone follows a diet and the outcomes are based on that nutrients or lack of nutrients that come from that diet. To achieve personal improvements in health an individual must alter their diet to reflect a diet that better suites their body. For example; if an individual struggles with weight gain and weight related disease then their diaeta or way of life is not leading that individual to life of poor health. To change that future the individual can simply change their diet or diaeta to better provide the nutrients their body thrives on.
The key to a successful diet is understanding that it is a way of life. Understanding that these changes need to be life long changes to continue to support optimal health for your body. First, identify the diet that provides you with the desired health and longevity you want, then keeping that diet consistent to continue to support and achieve those goals. That is the simple truth of a successful diet, keep it simple. The hard part is how people over complicate the process.