The reaction where oxygen is removed from a chemical is called reduction.

Here's why:

* Reduction is one half of a chemical reaction called redox (reduction-oxidation).

* Oxidation is the process where a chemical loses electrons.

* Reduction is the process where a chemical gains electrons.

Since oxygen is highly electronegative (meaning it attracts electrons), when it's removed from a compound, the other element involved gains electrons, thus being reduced.

Example:

* In the reaction of copper oxide (CuO) to copper (Cu), oxygen is removed from the copper oxide, which is reduced to copper:

CuO + H₂ → Cu + H₂O

Here, copper oxide is reduced to copper, while hydrogen is oxidized to water.