Definition of nutrient density.

Nutrient density is the sum of nutrients a food contains in proportion to its calories.

For example, when a food has a substantial concentration of different nutrients (e.g., vitamins, minerals, proteins) and is also low in calories, we note that this food has an increased nutrient density (i.e., it offers enhanced nutrition with a minimum amount of calories).

The nutrient density of some foods:

Olives-Olive oil.

Olive oil is perhaps a characteristic example where a decrease in the nutrient density of the actual food (olive) occurs during the production process.
Separating the olive's lipidic phase removes the solid and water-soluble components, resulting in an adverse change in the final concentration of many minerals and water-soluble vitamins.
Thus, although initially, ten medium olives could supply us with 0.6g. fiber, 2.5mg Iron, 35mg Calcium, and 0.3g. protein, these concentrations in the produced olive oil are nearly negligible.

Sugar-Raisins.

In addition to its zero nutritional value, sugar can burden our bodies with unnecessary calories. For example, if we decided to add 100gr. of raisins instead of 100gr. of sugar in any recipe, we would obtain substantial concentrations of Iron, Potassium, and Copper instead of the nearly zero concentration of these minerals in processed sugar. In this instance, we can state that raisins have a higher nutrient density than processed sugar.
Σε αυτό το παράδειγμα οι σταφίδες προφανώς υπερτερούν σε θρεπτική πυκνότητα σε σύγκριση με την επεξεργασμένη ζάχαρη.

Lettuce, or kale.

The humble lettuce is a misunderstood vegetable with increased nutrient density. Specifically, due to its low anti-nutrient content (its low concentration of phytic acid), it contains fully absorbable minerals such as Calcium, Iron, and Magnesium. Furthermore, it can also be a rich source of omega-3 polyunsaturated fatty acids (ALA).

The nutritional value of kale has become well known nowadays. Due to its low oxalic acid concentration, its mineral content is more bioavailable. Specifically, a biochemical study reveals that its high intestinal absorption of Calcium exceeds that of animal milk.

Is energy density an ultimate goal for everyone?

Finally, it should be noted that achieving the nutrient density of foods shouldn't be a conscious concern for all of us.
For example, marathon athletes who may consume 5000-7000 calories daily for their training are not particularly interested in reaching a high nutrient density in their meals.

Instead, they mainly desire a high energy density, i.e., high-calorie foods with plenty of carbohydrates (such as fruits, juices, and energy drinks), to replenish their lost glycogen stores and fulfill their potential athletic goals.

Sources

Calcium absorption in kale:
https://pubmed.ncbi.nlm.nih.gov/2321572