What term describes a decrease in speed or negative acceleration?

The term that describes a decrease in speed or negative acceleration is deceleration. In physics, acceleration refers to any change in velocity, which includes both increases and decreases in speed. When an object is slowing down, it is experiencing negative acceleration, specifically referred to as deceleration. This term effectively communicates the idea that the object's speed is being reduced over time.

Acceleration is a more general term that can mean an increase or decrease in speed. Velocity specifically refers to the speed of an object in a given direction and does not inherently convey whether the speed is increasing or decreasing. Momentum refers to the quantity of motion an object has, which is dependent on both its mass and velocity, and does not describe the change in speed. Thus, deceleration is the most accurate term for describing a reduction in speed.