What is an "Indian Summer", and why is it so-named?

(this quoted directly from the Meteorological Glossary, HMSO): " A warm, calm spell of weather occurring in the autumn, especially in October and November. The earliest record of the use of this term is at the end of the 18th century, in America, and it was introduced into the British Isles at the beginning of the nineteenth century. There is no statistical evidence to show that such a warm spell tends to recur each year. "

C.E.P. Brooks, in his 'Climate in everyday life', notes that it is the counterpart of our 'Old Wives Summer', here in Europe, and tends to follow the first severe frost and to persist for several days.

It is thought that the phrase was coined by European settlers on the Atlantic coast of North America. Paul Marriott, in his 'Red Sky at Night, Shepherd's Delight', says..." strictly an Indian Summer is a lengthy dry sunny spell from late September into November. The name probably derived from the N. American Indians who relied on a similar fine spell in late autumn for harvesting. " Philip Eden, in his 'Weatherwise' (see the 'books' section of this FAQ) also ascribes this reasoning to the term. However, I have been advised of a belief in the USA that the phrase may be a rather pejorative one coined by the early settlers, which implies that a late ( autumnal / 'Fall' ) spell of warm, sunny weather is not to be relied upon: they found the native inhabitants (in their view) were not to be trusted in like fashion.

(There is yet another theory of the origin: Merchant vessels plying the Indian Ocean would have one of the 'load-lines' marked "IS" (for Indian Summer), to show the maximum load level for ships crossing that ocean in the post-monsoon fine weather season in the latter part of the year. It has been suggested that this might be the origin of the term; I have difficulty with this. The phrase can be traced back to at least 1778, yet the common marking of vessels in this way was not standardised until 1875 (Samuel Plimsoll, MP suggested the famous 'Plimsoll Line'). It is also difficult to see why the term should come to be associated with a phenomenon in North America.)

Such spells of fine, warm dry weather may be 'reliable' in the Atlantic states of the USA; this is not so for our own climate, and Marriott (amongst others) found expectation of a period of such weather in the U.K. to be misplaced.