What is considered a terminal illness for insurance?

The conditions that are covered will vary by insurer. Typically, it's a condition that can't be cured and is likely to lead to death. You can expect to see things like advanced cancer and heart disease.

Can you get critical illness cover after having cancer?

This would depend on the insurer, but you'll likely find it very difficult. It'd be classified as a pre-existing condition and you'd have to disclose it. It could be worth discussing your options with a broker or financial advisor.

Will life insurance pay out for terminal cancer?

If your life insurance policy includes cover for critical illness, which is sometimes known as terminal illness cover or trauma cover, then yes - it should pay out for a terminal cancer diagnosis.