An Indian summer is a period of unseasonably warm, dry, and calm weather occurring in autumn, typically after the first frost, in temperate regions of the northern hemisphere. It usually happens in October or November, though some definitions extend it to late autumn or early winter. The warm spell is often accompanied by hazy or smoky atmospheric conditions.

The term originated in the United States in the late 18th century, with the earliest known reference around 1778 by J. Hector St. John de Crevecœur, who described a warm interval after the first frost, calling it "Indian summer". The name likely relates to Native American practices of hunting during this warm period or the association with regions inhabited by Native Americans.

In addition to its meteorological meaning, "Indian summer" is used metaphorically to describe a late flourishing or revival, such as a period of renewed success or happiness late in life or at the end of a career. In British English, the term has been used since the early 19th century and is synonymous with similar European expressions like "Saint Martin's summer".

In summary, Indian summer denotes a brief, warm, and pleasant spell of weather occurring after the onset of autumn chill, often seen as a last warm respite before winter sets in.