straight line vs radius

straight line and radius both are nouns.

Word	Noun	Adjective	Verb	Adverb
straight line	Yes	No	No	No
radius	Yes	No	No	No

As nouns, radius is a hyponym of straight line; that is, radius is a word with a more specific, narrower meaning than straight line:

straight line: a line traced by a point traveling in a constant direction; a line of zero curvature
radius: a straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere)

Other hyponyms of straight line include perpendicular, asymptote, tangent, secant, diameter, chord, element, bias, diagonal, diagonal, vector.

straight line (noun)	radius (noun)
a line traced by a point traveling in a constant direction; a line of zero curvature	support consisting of a radial member of a wheel joining the hub to the rim
	the length of a line segment between the center and circumference of a circle or sphere
	the outer and slightly shorter of the two bones of the human forearm
	a circular region whose area is indicated by the length of its radius
	a straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere)