What is a reduction reaction? Give its examples.

Reduction is defined as the removal of oxygen or an electronegative element from a substance,or the addition of hydrogen or an electropositive element to a substance.
Examples:
$1. \text{ Removal of oxygen: } 2HgO_{(s)} \rightarrow 2Hg_{(l)} + O_{2(g)}$
$2. \text{ Removal of electronegative element: } 2FeCl_{3(aq)} + H_{2(g)} \rightarrow 2FeCl_{2(aq)} + 2HCl_{(aq)}$
$3. \text{ Addition of hydrogen: } CH_{2}=CH_{2(g)} + H_{2(g)} \rightarrow CH_{3}-CH_{3(g)}$
$4. \text{ Addition of electropositive element: } 2HgCl_{2(aq)} + SnCl_{2(aq)} \rightarrow Hg_{2}Cl_{2(s)} + SnCl_{4(aq)}$