negative acceleration meaning

Negative acceleration means the acceleration vector points opposite to the positive direction you picked on your diagram—sign convention, not a character flaw.

If you’re moving in the positive direction and acceleration is negative, you slow down; if you’re moving in the negative direction and acceleration is negative, you speed up (just more “backwards”).

“Deceleration” specifically means acceleration that reduces speed, while “negative acceleration” might reduce or increase speed depending on which way you’re travelling.

Units are still m/s², the physics is still merciless, and your brakes still prefer you choose a direction before you pick a fight with Newton.