Medically, a symptom is something the patient reports to you; in other words, a subjective impression by the patient. Nausea or pain are symptoms, since they are experienced by the patient and are not directly measurable by the observer.
Symptoms are very important for making diagnoses; unfortunately, many people are not good reporters of their symptoms.
Symptoms also cause the most headaches (aha, another symptom!!) in the Emergency Room, since you have to take the patient's word for it that they are, in fact, experiencing these problems. Symptoms are easy to fake with just a tiny bit of knowledge, and account for many ER visits. The difficulty is determining which patients are actually in terrible pain, and which are simply trying to get some Lortabs or Percocet. An often used indicator is how many pain medications the patient claims to be allergic to (also not an infallible tool, since there are people who legitimately ARE allergic to everything under the sun), which often is a way of the addict's trying to channel the MD into prescribing what he/she is after.