Hallo, Fizzy
My understanding of the difference is:
IDIOT - person of low intelligence ("mentally subnormal", very low IQ, brain function impaired by injury or disease such as alcoholism, Alzheimer's, syphilis etc)
IMBECILE - person with very limited or no communication ("severely mentally handicapped", severe brain damage possibly existing since birth, conditions such as low-functioning autism, possibly deaf/mute)
Was your relative on censuses before 1871 and, if so, what was his/her condition?
All the best
Philip