In the context of retro gaming, stands for "One Game, One ROM". Historically, full romsets for consoles like the Neo Geo CD
The "new" preservation standard for Neo Geo CD is (Compressed Hunks of Data). CHD reduces the 15GB full set down to ~5GB with zero data loss. All modern emulators (RetroArch, NeoCD, MAME) support CHD. 1g1r redump snk neo geo cd new