The establishment of a forest by artificial methods, by planting or seeding on land where trees have not historically grown (at least not for 50 years). Planting of new forests on lands that have not contained forests before, originally for hunting.

Wikipedia definition:

is the establishment of a forest or stand of trees (forestation) in an area where there was no previous tree cover. Many government and non-governmental organizations directly engage in programs of afforestation to create forests, increase carbon capture. Sometimes special tools, such as a tree planting bar, are used to make planting of trees easier and faster.

Source: Wikipedia - Afforestation