AspectRatio is an option for Show and related functions which specifies the ratio of height to width for a plot.
AspectRatio determines the scaling for the final image shape.
AspectRatio -> Automatic determines the ratio of height to width from the actual coordinate values in the plot.
The default value AspectRatio -> 1/GoldenRatio is used for two-dimensional plots. AspectRatio -> Automatic is used for three-dimensional plots.
See The Mathematica Book: Section 1.9.3, Section 1.9.7, Section 2.9.2 and Section 2.9.4.
See also: BoxRatios, PlotRegion.