Most of the policies have their own definition of a pre-existing condition which you can find in the brochure. Below however is a general definition:
Pre-existing Condition(s) are any medical condition, sickness, Injury, Illness, disease, Mental Illness or Mental Nervous Disorder, regardless of the cause including any congenital, chronic, subsequent, or recurring complications or consequences related thereto or resulting therefrom that with reasonable medical certainty existed at the time of application or any time during the “x” months prior to the effective date of coverage under this policy, whether or not previously manifested, symptomatic, known, diagnosed, treated or disclosed.
Pre- existing conditions are typically excluded. There are some policies (e.g. Atlas America, Patriot America, Liaison Continent, Inbound USA, Visitors Care etc.) however which can provide some benefits for an acute onset of a pre- existing condition.