A lightweight polyline (lwpolyline) is defined in the drawing database as a single graphic entity unlike the old-style polyline, which is defined as a group of subentities.
Lwpolylines display faster and consume less disk space and RAM. As of AutoCAD Release 14, 3D polylines are always created as old-style polyline entities, and 2D polylines are created as lwpolyline entities, unless they are curved or fitted with the AutoCAD PEDIT command. When a drawing from an earlier release is opened in AutoCAD Release 14 or a later release, all 2D polylines convert to lwpolylines automatically, unless they have been curved or fitted or contain extended data (xdata).