PREFIX
ac - to, toward, near
de - away, down
di - apart, away, not
pro - for, before, forward
re - back, again

ROOT
celer - fast
gress - step

SUFFIX
-ate - to make, to act
-ion - state, quality, act

WORDS AND DEFINITIONS
accelerate - to increase the speed of
decelerate - to reduce the speed of
digression - a departure from the main issue, subject, etc.
progress - a movement forward or onward; improvement
regression - a movement backward to an earlier state