Pre-existing conditions are medical conditions that exist at the time you arrange your insurance.
Different insurance policies will contain different definitions of a pre-existing condition. The definition can include a symptom, a sign, or a circumstance of the condition being present at the time you take out the policy, rather than requiring a diagnosis of the condition itself. That means a symptom can be a pre-existing condition, even if you do not have a specific diagnosis. Check your policy to understand the definition that applies to you.