* Acceleration is the rate of change of velocity. Velocity has both speed (how fast you're going) and direction.
* Zero acceleration means your velocity isn't changing.
* Constant speed in a straight line means your velocity isn't changing, both in terms of how fast you're going and your direction.
In other words: If you're moving at a steady pace in a straight line, you're not speeding up or slowing down, and you're not changing direction. This means your acceleration is zero, and your speed is constant.
