There is not a single definition of pain that is appropriate for everybody because it is a highly subjective experience. What, to another person, is excruciating may be nothing more that a slight discomfort for you. Not only do views of pain vary among individuals, your own perception of it can change over time.
Even when you do have a clear perception of what pain means to you, there is no objective way to measure it you can use to convey your impressions to somebody else. It is not unusual for patients in a doctors office, who have come in because pain is severely impairing their day-to-day lives, to have great difficulty describing it clearly.
One thing we do know, however, is the difference between when we are hurting and when we are not. In the case of acute pain, you may cry out from it and experience terrible suffering for a time, but it ends eventually, and usually the sufferer returns to their normal activities and way of life.
Chronic pain is different. A sufferer of chronic pain not only hurts, but they keep on hurting. Indeed, the psychological impact of chronic pain can be worse than the physical sensation itself, especially when the daily grind of it wears you down and turns the world grim. Acute pain usually does not change ones personality. Chronic pain, if not addressed properly, can alter it drastically.
Chronic pain may have a physical basis, a psychological basis, or some mix of the two. Maybe it comes from an injury. Maybe it comes from stress. Maybe the two factors are interconnected. People who cannot pinpoint a clear physical connection often say, or are told, that it is all in their heads.
But that is not how it feels. Chronic back pain caused by stress can hurt just as badly as chronic pain that resulted from falling off of a roof.
It has been estimated that over 34 million Americans suffer from chronic pain, be it from arthritis, migraine headaches or their backs (with lower back pain being the most common). 15 million people experience chronic pain at work on a daily basis. So if you are a sufferer of chronic pain you are not alone though it can certainly feel that way.