| Agent | Background service that performs Defrag operations and manages schedules. |
| Allocation size | Space actually allocated to a file on disk; it can differ from logical size. |
| Analysis | Reading and evaluating a drive's layout without reorganizing it. |
| Automatic optimization | Background maintenance using configured intervals. |
| Boot-time optimization | One-time automatic or manual maintenance in Fortress Mode, started now or scheduled for the next system start. |
| Cluster | A file-system allocation unit. |
| ClusterView | Visual map of a volume's clusters, compressed into screen blocks. |
| ClusterInspector | File information for a selected area in ClusterView. |
| COMPLETE | Family of methods that reorganize HDD files by name or timestamps. |
| Defragmentation | Joining separately stored extents of files. |
| Exclusion | A path or mask specifying files that should be skipped. |
| Extent / fragment | A contiguous run of stored file data. |
| FileInspector | Filtered file lists used to inspect drive usage and fragmentation. |
| Fortress Mode | O&O Defrag's dedicated Windows RE maintenance environment. |
| Free-space consolidation | Joining separate free regions into fewer areas. |
| HDD | Hard disk drive with mechanical storage. |
| Inclusion | A rule forcing matching files into defragmentation selection, overriding exclusions. |
| Job | A saved maintenance task with drives, methods, schedule, and conditions. |
| Logical size | The size of a file as seen by applications. |
| Metadata | File-system structures used to describe and manage stored data. |
| OODCMD | O&O Defrag's end-user command-line tool. |
| OPTIMIZE | Family of optimization methods requiring configured zones. |
| ROI factor | Benefit of reducing fragmentation relative to required writes. |
| S.M.A.R.T. | Device-provided health and monitoring information. |
| SOLID | File optimization designed to limit writes; the CLI calls the quick variant SOLID/Quick. |
| SOLID/Complete | More thorough SOLID optimization with additional consolidation. |
| SPACE | Method used for free-space consolidation goals. |
| SSD | Solid-state drive using flash storage. |
| STEALTH | Defragmentation method emphasizing low resource use. |
| TRIM | Notification to an SSD that specified storage regions are no longer needed. |
| Volume | A logical Windows storage volume, commonly identified by a drive letter. |
| Windows RE | Windows Recovery Environment used to prepare boot-time maintenance. |
| Zone | File group with placement rules and an optimization method. |