A "Charley Horse", or "Charlie Horse" is the informal name for a cramp, spasm or feeling of stiffness in the arm or leg region. Typically, this pain does not last longer than a few seconds or minutes.
Spasm: a sudden involuntary muscular contraction or convulsive movement -- Cramp: a painful, involuntary contraction of a muscle or muscles, typically caused by fatigue or strain
Soccer players experiencing muscle cramps.
The colloquial name is just used in America. The name was thought to originated in the late 1800's in baseball, where MLB player Charley Radbourn often experienced cramps. One other potential origin is from 1700's foot patrol police officers, called Charley's, who often had aches in their legs from walking.
The main symptom, or feeling before screaming, "Ah, I have a Charley Horse!" is a tight feeling within the muscle, whether that be in the leg (more common) or arm.
The root of these cramps are often relates back to low potassium or calcium levels, dehydration, or old age.
As for a diagnosis, doctors can: identify hard or tight muscle areas, check patient for Restless Leg Syndrome (usually more intense syndromes like an unpleasant tickling/ twitching sensation in the leg muscles when sitting or lying down, only relieved by movement). For the most part Charley Horse's are self-treatable and not something to be worried about.
Treatment and prognosis of Charley Horse's include stretching, massages, heat/ ice rotation, resting in different positions, changing one's diet to balance mineral deficiencies and staying hydrated.