Constitutionally, to what does the term incorporation refer?

The term incorporation refers to constitutional doctrine.

Explanation:

The first ten amendments of the USA are made applicable to the public through the constitutional doctrine. The supreme court decides to incorporate certain selected amendments rather than incorporating the entire amendment, this is called selective incorporation.

Although the ninth and tenth amendments are not incorporated and will never be and the ninth amendment interacts with the state law, on which the supreme court relies on while deciding cases. Also, before this doctrine, the Bill of rights was applicable only to the federal government and the court cases.