Critical Illness policies pay a lump sum on diagnosis
of specified medical condition examples are 
 
cancer
coronary artery by-pass surgery
heart attack
kidney failure
major organ transplant
multiple sclerosis
stroke
aorta graft surgery
benign brain tumour
blindness
coma
deafness
heart valve replacement or repair
loss of limbs
loss of speech
motor neurone disease
paralysis/paraplegia
Parkinson's disease
terminal illness
third degree burns
 
Please note the exact definitions and conditions covered will vary between insurers.