How do health insurance companies define pre-existing health conditions?

Is it based on the time of diagnosis or the time the condition was likely to have originated? For example, what if I take out health insurance and a week later I go to the doctor and get diagnosed with cancer? I didn't know about it beforehand but the insurance company says the lump must have taken months to develop? What's the outcome?
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A pre-existing condition is a health condition that existed before the policy was purchased. Generally, for both group and individual insurance plans, if you have seen a physician within the previous 12 months (this period may vary in different plans) for a health condition, that particular condition will not be covered, either permanently or for a specified waiting period (usually six months to two years), or you may be refused coverage or charged a higher premium for the coverage. The National Coalition for Cancer Survivorship (NCCS) points out that cancer survivors have a pre-existing condition from the time of diagnosis through the remainder of their life, since a cancer survivor usually needs to see a physician at least once a year for a checkup.
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