Definition
Pathological describes something caused by or involving disease, or something that is extreme and not normal. Unlike something merely unhealthy, 'pathological' suggests a condition that is beyond the ordinary range of variation. It can apply to physical conditions, like a tumor, or to behaviors, like pathological lying. In the latter case, it suggests a compulsive and abnormal pattern. A pathological liar, for instance, can't help but lie, even when there's no reason to. This indicates a deeply ingrained issue.