Cost Centres is the default term for an optional additional top-tier of classification that can be used in addition to Projects and Tasks. Cost Centres are disabled by default but can be enabled via System Configuration. Once enabled values can be recorded when entering time in the time entry applications.
If you prefer to use a different term than "Cost Centre" then you can change this configurable text.
Cost centres marked as inactive can no longer be selected but will still be available for selection when reporting on time.