The word «nutrition» refers, in its most traditional sense, to the process through which the body separates food to obtain substances (named «nutrients») that it will use for numerous functions. That process consists principally of three phases: digestion (separation of food into nutrients), absorption (passage of nutrients from the digestive tract into the blood) and transport (move of nutrients, through blood, to the place where they will have an effect).
But the word «nutrition» refers, in its most correct sense, to the science that studies that process more broadly (from food outside the body, to its last effect in it after ingesting it) and everything that has a certain relationship with it. Nutrition is an extremely complex science, that interacts with other sciences and numerous factors. It can be considered a new science (because it began to be investigated more thoroughly, relatively recently) about which there is still much to investigate, and about which population (in general) has much ignorance.
The effect of food in the body, depends on a lot of causes, and can be positive or negative. Those consequences tend to be multifactorial and noticeable in the long term, so (added to the ignorance of population, in general) it is not usually given (to nutrition) the importance that it deserves. Due to the different effect depending on what is ingested, how it is ingested, when it is ingested,… feeding should not be random, but systematic and personalized.