Indian Summer
2 of the 7 encyclopedias on this shelf carry an entry for Indian Summer. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
in the United States and Canada, a name applied by general consent to a hazy, smoky, warm spell of weather expected late in autumn. The cold weather which precedes Indian summer is called squaw winter. So far as the origin can be traced, the term appears to have arisen in western Pennsylvania about 1790, but the exact origin is not known. In the West the term is supposed to indicate the season when the Indian went hunting for buffalo and other game to lay in his winter's meat. The Dakota Indians, at least, counted on a time of this sort along in October and November when fine hunting weather might be expected. Whatever its origin, the term is now well set in literature. A line or two from Lowell's An Indian Summer Reverie is expressive of the season's rare beauty: What visionary tints the year puts on, When falling leaves falter through motionless air Or numbly cling and shiver to be gone! How shimmer the low flats and pastures bare. How fuse and mix, with what unfelt degrees, Clasped by the faint horizon's languid arms, Each into each, the hazy distances! The softened season all the landscape charms; Those hills, my native village that embay, In waves of dreamier purple roll away, And floating in mirage seem all the glimmering farms. From gold to gray Our mild, sweet day Of Indian summer fades too soon; But tenderly Above the sea Hangs, white and calm, the hunter's moon. In its pale fire The village spire Shows like the zodiac's spectral lance; The painted walls Whereon it falls Transfigured stand in marble trance! --John Greenleaf Whittier.
Collier's New Encyclopedia (1921)
the name given to a period which generally occurs towards the end of mild summer weather of autumn in the United States.