Rationalise meaning : To make something rational or more rational. To justify an immoral act, or illogical behaviour. “The process of thought by which one justifies a discreditable act, and by which one offers to oneself and the world a better motive for one's action than the true motive” To remove radicals, without changing the value of an expression or the roots of an equation. To structure something along modern, efficient and systematic lines, or according to scientific principles. This often includes eliminating duplication and grouping like or similar items.