The act or process of reducing. The result of reducing: a reduction in absenteeism. The amount by which something is lessened or diminished: a reduction of 12 percent in violent crime. Biology The first meiotic division, in which the chromosome number is reduced. Also called reduction division. Chemistry A decrease in positive valence or an increase in negative valence by the gaining of electrons. Chemistry A reaction in which hydrogen is combined with a compound. Chemistry A reaction in which oxygen is removed from a compound. Mathematics The canceling of common factors in the numerator and denominator of a fraction. Mathematics The converting of a fraction to its decimal equivalent. Mathematics The converting of an expression or equation to its simplest form.