Which term do biologists use to describe the average number of individuals of a species per unit area?

Population density is the right term.

Population density of any species (plant, animal or humans) is the number of individuals found in a given unit of area. It is usually expresses as “X” species in “Y”

X  = number of species

Y = unit of area , it can be square feet, Square meter, Square kilometer, Square yard etc.  

For example , population of 1000 insects living in an area fo 1000 square feet has a population density of 1000/1000 insects per square feet = 1 insect per square feet.

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