The following terminology is used when shutting down and booting a system:
A run level is a letter or digit that represents a system state in which a particular set of system services are available. The system is always running in one of a set of well-defined run levels. Run levels are also referred to as init states because the init process maintains the run level. System administrators use the init command or the svcadm command to initiate a run-level transition. This book refers to init states as run levels.
A boot option describes how a system is booted.
Different boot options include the following:
Interactive boot – You are prompted to provide information about how the system is booted, such as the kernel and device path name.
Reconfiguration boot – The system is reconfigured to support newly added hardware or new pseudo devices.
Recovery boot – The system is hung or an invalid entry is prohibiting the system from booting successfully or from allowing users to log in.
For terminology that is specific to GRUB based booting, see x86: GRUB Terminology.