American English
American English refers to the dialect of the English language spoken in the United States of America. A popular sense of the term is that it means only the 'standard', typically written form of English in the USA. Linguists, however, would use it to mean any dialect, standard or not, that is used in America. Often the accents of the USA are included in the definition too.
Spelling conventions
One reason that 'American English' might be associated with standard written language is that one of the most obvious differences between this and other varieties of English is its spelling conventions: color rather than British English colour, for example. These 'American' spellings are also common in many other countries, particularly where the locally taught form of English is often the American variety: South Korea is one example.
Varieties of American English
American English is not a single dialect, similar for all speakers throughout the USA and its insular areas. It differs from region to region and often within the same towns, cities and local communities, though it is often claimed that it shows less diversity than might be found elsewhere in the English-speaking world, particularly when discussion refers to the western USA.
Is 'American English' also 'Standard English'?
'Standard American English' - meaning the formal variety of American English which is the subject of dictionaries and grammar books, and is taught in schools and to learners of English in the USA and in various places overseas - is just one form of American English today. Linguists would consider its study highly valuable, but other dialects would be equally worthy of consideration. Confining the definition of 'American English' to the standard variety also creates a problem: all forms of a living language change all the time, as new generations develop new vocabulary or reanalyse one aspect of their native language's grammar in a slightly different way from how their parents used it. This also applies to the 'standard' language, though this process is typically slowed by its being codified in a set of written conventions and prescriptivist grammar rules. If linguists were to agree with the popular definition of American English as standard English, new innovations would go uncatalogued and linguistic diversity could not be used as data to further understanding of language itself.