The  medical term [symptom] has many definitions, as shown in this article. A good definition follows: A symptom is that what a patient refers, it is subjective, and cannot be measured or standardized (between patients). Examples of symptoms are thermal sensation ("I feel hot or feverish"), visceral sensations ("I feel a lump on my throat"), etc. In all cases the main characteristic of a symptom is that is subjective and patient-dependent.  It is only relevant to a particular patient and cannot be compared from one patient to another.

Symptoms are part of the clinical study or history of a patient that help lead to a diagnosis.

• Merrian-Webster: "subjective evidence of disease or physical disturbance", "something that indicates the existence of something else"
• Memidex: "any sensation or change in bodily function that is experienced by a patient and is associated with a particular disease"
• Pandora World: "Feelings and perceptions reported by a patient   indicative or that can be correlated with a disease process"
• Oxford Dictionaries: "A physical or mental feature that is regarded as indicating a condition of disease, particularly such a feature that is apparent to the patient"

Personal note: Pain is a subjective entity, as it is characteristic to an individual. We all feel and respond to pain differently, as there are people with higher and lower thresholds to pain. Although I understand the need to have some type of standard, I dread the question posed by many..."From one to ten, can you tell me what your pain level is?.  If the answer allows the physician or nurse to compare levels of pain within the same patient and see the evolution of a pathology, I am OK with that. But you cannot use that measurement to compare pain levels between patients!... at least that is my opinion. Dr. Miranda