Serious illness cover, also known as specified illness cover, pays a lump sum if you are diagnosed with one of the illnesses listed in the policy. You choose how to use it: clear debts, replace income while you recover, pay for treatment or adapt your home.
The illnesses covered, and how each one is defined, vary from insurer to insurer. One insurer may pay in full on a condition another pays only in part. That is why we compare the definitions, not only the price.