Rationaali

Rationaali is a term used primarily in Finnish mathematics and linguistics to denote concepts related to rationality and rational numbers. In the context of mathematics, a rationaali refers to a rational number, that is, any number that can be expressed as the ratio of two integers, with the denominator non‑zero. Rational numbers include whole numbers, fractions, and terminating or repeating decimal expansions. They form a field under the usual operations of addition, subtraction, multiplication, and division (excluding division by zero), and they are dense in the real number line, meaning between any two distinct real numbers there exists a rational number.

In number theory, properties of rationaali numbers are studied with respect to their representations, uniqueness of reduced forms, and their behaviour under arithmetic operations. Rational numbers are contrasted with irrational numbers, which cannot be expressed as a quotient of integers and exhibit non‑repeating, non‑terminating decimal expansions. The set of rational numbers is countably infinite, whereas the set of real numbers is uncountably infinite.

Outside mathematics, the Finnish word rationaali also serves as a general adjective meaning "rational" or "reasonable," used to describe sound reasoning or decisions. In this usage it parallels the English term rational and is often applied in philosophical discussions, economics, and everyday speech to denote logical or practical justification.