A package without a lid (which refers to the package cover of the device).

A lidless package has a much lower ΘJC than a package with a lid because the junction-to-case distance is shorter.

For details, please refer to the document at the URL link below.

Package Information Datasheet for Altera Devices
https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/ds/pkgds.pdf

AN659: Thermal management and mechanical handling for lidless flip chip ball-grid arrays
https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/an/an659.pdf