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