Dihydroergotamine is used to treat migraine headaches and cluster headaches. It is not recommended for migraines that affect only one side of the brain (hemiplegic migraine) or the base of the brain /neck area (basilar migraine), or to prevent migraines from occurring.
Pop music is an abbreviation for popular music. In other words, the main difference between pop music and alternative or underground music is the kind of following it has: pop music is usually known by a lot of people and regularly appears on TV.
Pop is short for popular, and it's remained the defining term for the ever-changing music favoured by the public. Although not specifically applied until the middle of the 20th century, pop music as such can be traced by a few decades before that.