Free Radical: a chemical compound that contains an unpaired valence electron (an odd number of valence electrons).  Such substances are highly reactive, since their unpaired valence electron will react rapidly with other electrons in adjacent molecules.  Free radicals can be formed by the absorption of high energy ionizing radiation (such as ultra-violet light).  In a cell, a free-radical can react with DNA, causing potential genetic damage.  Free-radicals are responsible for many other chemical reactions, including bleaching.   Free radicals can only be destroyed by reaction with another free radical.

Forward reaction: the reaction in the "traditional" direction, that is from left to right for the reactants turning into products. The opposite of the reverse reaction.

Reactants     Products