The act of adding something to what is already there, so that the total reaches an acceptable level. It is the noun for the action of supplementing.

By far its most common home today is health and nutrition: vitamin supplementation, iron supplementation, calcium supplementation. Here it means giving people a nutrient they are not getting enough of from food. "The study looked at the effects of vitamin D supplementation in older adults." Doctors, nutrition researchers and health articles use it constantly.

It also appears in economics and farming, where the level being topped up is money or animal feed — income supplementation, feed supplementation — but these are much less common.

This is a heavy, formal, written word. Almost nobody says it in conversation. In speech an English speaker would say "taking vitamin D" or "taking supplements", not "vitamin D supplementation". So recognise it when you read research and health writing, and choose the simpler wording when you speak.

Notice the pattern: the noun is usually followed by nothing at all, and the thing being added goes in front of it — iron supplementation, not "supplementation of iron" (though "supplementation with iron" does occur in medical writing).