This term refers to base unit of budgeting.  It was selected as a neutral definition because organizations use varies terms including Departments, Cost Centers etc.

OrgUnits are defined in System Management / Companies / Manage Company Databases

*Do not attempt to change an Object or Orgunit configuration without first consulting Dynamic Budgets support, The system may need to be reloaded entirely. 

Related Articles include Manage OrgUnits, Row Format, and Understanding Reporting Levels.