[rightbyte]

More stringently, deceleration is decreasing the magnitude of the velocity vector, I would say.

If acceleration can be negative, so can speed. A negative speed with negative acceleration would not imply deceleration?

[DiogenesKynikos]

The magnitude of the velocity vector is dependent on the frame of reference.

If you measure the same object's velocity from a spaceship traveling through the solar system, you'll get a different answer from what we measure from Earth.

That's why physics doesn't distinguish between acceleration and deceleration. What looks like acceleration in one frame looks like deceleration in a different frame.

[mongol]

Speed is not a vector, it is a scalar. You are thinking of velocity.