What does it mean when chemists say that a reactant is a reducing agent?

Question

What does it mean when chemists say that a reactant is a reducing agent?

Answer 1

Now this is complicated because we have to think backwards, The opposite of "reduced" is "oxidised". Oxidation is the gaining of electrons.
In a redox reaction, these go hand in hand.

A reducing agent is something that can cause reduction. If a reactant is a reducing agent, it does the reducing of another reactant. It therefore is a substance that can donate electrons. In reducing another reactant, the reducing agent loses electrons and so it is oxidised.