Which concept in physics describes the phenomenon by which an object accelerating in a fluid experiences a force opposite to its direction?

Air resistance

Air resistance, also known as drag, plays a crucial role in the design of vehicles, aircraft, and even in sports. This force, opposing the direction of an object's movement in a fluid (like air or water), increases with the object's speed, which explains why racing cars or airplanes have such aerodynamic shapes to minimize this resistance. Understanding and optimizing air resistance has led to significant improvements in energy efficiency and performance across many fields. Without this knowledge, high-speed travel, whether on land or in the air, would be much more difficult and consume far more energy.