A pre-existing condition is any injury, illness, or condition the plaintiff had before the incident in question. Insurance carriers love pre-existing conditions because they argue that the current pain or limitation is not from the crash but from the prior issue.
The legal answer: defendants take plaintiffs as they find them (the 'eggshell plaintiff' rule). If a pre-existing condition was aggravated by the new injury, the aggravation is compensable. The plaintiff doesn't have to prove the new injury caused all current pain. Only that it made the existing condition worse.
Documentation is everything. Medical records pre-dating the accident establish the baseline. Records after establish the new state. The difference is the aggravation, and that's what the case is about.