To give you a dictionary definition, a pre-existing condition is a health condition that existed before a person’s health insurance policy came into effect. Some pre-existing conditions would be excluded for coverage under some health insurance policies because of the potential for adverse selection. Some pre-existing conditions would even make you ineligible for coverage completely. Although note that insurers in the United States must insure pre-existing conditions under the Affordable Care Act.
That being said, not all prior medical conditions are considered pre-existing conditions that would affect your health insurance coverage. For example, the flu or a knee injury are medical conditions but neither would count. While I cannot give you a full list here, material pre-existing conditions would be the more serious ones like HIV/AIDS, a previous heart attack, having had a major organ transplant, and Parkinson’s disease. Your health insurance provider will have more details on what they consider to be pre-existing conditions and you can also consult an insurance agent or broker about your particular situation (to learn more about the experts available to help you, see Medical Concierge or Insurance Broker: What’s the Difference?).
However, having one of these conditions does not automatically make you inelligible for health insurance. For example, with the Canadian health insurance provider Sun Life some conditions like the ones listed above would preclude you from applying for personal health insurance, but if you only have one pre-existing and condition and meet other eligibility requirements, you can still apply for their Health Coverage Choice insurance.