slow vs decelerate

slow is an adjective but decelerate is not an adjective.

Word	Noun	Adjective	Verb	Adverb
slow	No	Yes	Yes	Yes
decelerate	No	No	Yes	No

As verbs, slow and decelerate are synonyms defined as:

Other synonyms of slow include retard, slow down, slow up.

As verbs, decelerate is a hypernym of slow; that is, decelerate is a word with a broader meaning than slow:

Other hypernyms of slow include retard, slow, slow down, slow up.

slow (adjective)	decelerate (adjective)
not moving quickly; taking a comparatively long time
at a slow tempo
(used of timepieces) indicating a time earlier than the correct time
(of business) not active or brisk
slow to learn or understand; lacking intellectual acuity
so lacking in interest as to cause mental weariness

slow (verb)	decelerate (verb)
lose velocity; move more slowly	lose velocity; move more slowly
become slow or slower	reduce the speed of
cause to proceed more slowly

slow (adverb)	decelerate (adverb)
without speed (`slow' is sometimes used informally for `slowly')
showing a time that is earlier than the actual time