A ratio of the number of items within a defined unit of area measures

Density

Step-by-step explanation:

Density is the number of items per unit area. It is gotten by dividing the number of items by the area it occupies. It measures the compactness of an item in a given area. The higher the density, the more compact items are per unit area and the lesser the density, the less compact items are per unit area. It is measured in items/m²