Final answer:
Substance A, which receives an electron, is reduced. The process of gaining electrons in a chemical reaction is known as reduction, while the opposite process where a substance loses electrons is termed oxidation.
Step-by-step explanation:
When substance A receives an electron from substance B, substance A has been reduced. This is because reduction is defined as the gain of one or more electrons in a chemical reaction. Consequently, substance B, which has lost an electron, has been oxidized. In a balanced oxidation-reduction (redox) reaction, there is a clear transfer of electrons where the substance gaining electrons is reduced and the one losing electrons is oxidized.
For example, when aluminum (Al) is oxidized, it loses electrons, while silver ion (Ag+) in the reaction gains the electrons and is reduced. Similarly, in a reaction where oxygen gains electrons, oxygen is acting as the oxidizing agent and is reduced. The flow of electrons in such electrochemical processes is what characterizes a redox reaction.