Christian Dior’s A Line, presented in 1955, emphasized a narrow waist with a skirt that widened toward the hem.
The name describes the visual outline of the garments: the upper body is relatively fitted, while the lower body expands to form the shape of a capital A. Dior used letter names to make the changing geometry of his collections easy to identify.
The A Line followed the famous 1947 Corolle silhouette and was part of Dior’s continuing experimentation with proportion. It should not be confused with the H Line of 1954, whose shape was straighter, or the Y Line, which focused attention on broad shoulders.
Fashion histories sometimes use “A-line” as a general term for any skirt that widens toward the hem. In this question, however, the answer refers specifically to Dior’s named 1955 collection concept.