diametric

Diametric is an adjective that relates to a diameter, the straight line through the center of a circle or sphere that connects two points on its boundary. The term comes from Greek dia- “through” and metron “measure.” In geometry, diametric relationships describe things aligned along a diameter or involving its endpoints.

In circles, two points that lie at the endpoints of a diameter are called diametrically opposite. The distance between such points equals the circle’s diameter, and any line through the center that connects them is itself a diameter. The concept extends to three-dimensional spheres, where diametric pairs are the endpoints of a diameter drawn through the center.

In astronomy, diametric opposition describes a configuration in which two celestial bodies lie on opposite sides of the sky from the observer; for outer planets this is commonly called opposition, when the Earth lies between the Sun and the planet. The planet is generally brightest and most visible at night and can appear largest in apparent size during opposition, depending on orbital distances.

In everyday language, diametric often appears in the phrase diametrically opposed, meaning complete or absolute opposition of positions, opinions, or tendencies. The term is used to emphasize sharp contrast or fundamental difference between concepts or phenomena.

Related terms include diameter, diametrical, and diametrically opposed. While primarily geometric in origin, diametric also serves as a figurative device in discourse to convey oppositional relationships.