What is the difference between "slowing down" and "deceleration"? What is the difference between "speeding up" and "acceleration"? Which of these four phrases would apply to an object that goes from a velocity of -5m/s to -100000m/s?
'deceleration' means that the acceleration vector is directed oppositely to the instantaneous velocity vector -- it is a 'special case' of accelerating. When an object changes velocity from -5m/s to -100000m/s it has accelerated (considerably) -- the direction of acceleration is the same as the direction of motion. The following two sentences are completely equivalent:
Starting with a velocity of 10m/s, an object decelerates with a = -1m/s^2 until it comes to rest.
Starting with a velocity of 10m/s, an object subsequently accelerates with a = -1m/s^2 until it comes to rest.
I would say that 'slowing down' or 'speeding up' are not technical terms, or at least that they don't always simply relate to the concept of acceleration. Although there may be simple relationships between the terms for 1-dimensional motion, these won't hold up in more complicated situations. For example, in uniform circular motion the object is always accelerating, but never 'speeds up' -- the magnitude of the velocity vector is unchanged with time.