súde

Säde is a Finnish noun meaning a ray or beam, and in geometry also the radius of a circle. It denotes a line that starts at a point and extends outward (a geometric ray) as well as the fixed distance from the center of a circle to its edge (the radius). The word can also refer to a spark or flash in poetic or figurative language. In common usage, säde appears in phrases such as valon säde (a ray of light) and ympyrän säde (the circle's radius).

Säde originates in Finnic languages and has cognates in neighboring tongues, such as Estonian säde, with similar meanings of ray or beam. The term is a standard technical word in science, mathematics, and education, and it follows ordinary Finnish noun inflection.

Usage and nuance: In science, säde is used for both the radius of a circle and a light ray, depending on context. In everyday speech it can also convey metaphorical brightness or influence. The term is common in textbooks, engineering discussions, and literature, and it remains a core vocabulary in Finnish for light, geometry, and physical descriptions.