Redox reactions, or oxidation-reduction reactions, are a family of reaction that are concerned with the transfer of electrons between species. Oxidation typically refers to the loss of electrons while reduction refers to the gain of electrons. More precisely, oxidation is defined as an increase in the oxidation state of an element (more positive oxidation number) and reduction is a decrease in the oxidation state of an element (more negative oxidation number). Redox reactions are always a matched set, meaning that there cannot be an oxidation reaction without a reduction reaction happening simultaneously. The oxidation and reduction portions of a particular reaction can be written separately and are called half-reactions. When writing half-reactions, the gained or lost electrons are usually explicitly included so that the half-reactions can be balanced with respect to electric charge.