SortDirection is a generic term used in various technologies to specify the sorting direction of the items in a list, grid, etc.
WPF: DataGridColumn.SortDirection gets or sets the sort direction (ascending or descending) of the column. It is used to queue the visual arrow in the DataGridColumnHeader to point up, down, or to not show.
Windows Forms: BindingSource.SortDirection gets the direction the items in the list are sorted.
ASP.NET: GridView.SortDirection gets the sort direction of the column being sorted.