Question

when is a substance said to be reduced in a chemical reaction?

Answer 1

Well, when there is the gain of electron occur in the chemical reaction the substance which has gain the electron is said to reduced one. 
The gain in electron means the change of oxidation will also occur.
This also means the gain of hydrogen or loss of oxygen occur in the molecule.
The gain of electron is more precise thing in order to understand this , there could be gain of one electron or more than one electron.

Actually reduction  reaction normally seen from the point of view of redox reaction where there is one substance reduced while the other oxidized

Answer 2

Very Interesting question:Thanks for asking it.

In a chemical reaction, if oxygen is removed or Hydrogen is getting added then we call the equation as reduction reaction.

Cu2O + H2 ----->2Cu + H2O

Here cuprous oxide is said to be reduced as oxygen is removed.
The reduction agent is H2.