• Active Time – The period of time during which an item is in an operational inventory.
• Inactive TIme – The period of time during which an item is in reserve.
• Uptime – The period of time during which an item is in reserve. The period of time during which an item is in a condition to perform a required function.
• Downtime – The period of time during which an item is not in a condition to perform a required function.
• Maintenance Time – The period of time during which an item is not in a condition to perform a required function.
• Modification TIme – That part of downtime necessary to introduce any specific change(s) to an item to improve its characteristics, or to add new ones.
• Delay Time – That part of downtime during which no maintenance is being accomplished on the item because of either supply or administrative delay.
• Supply Delay TIme – That element of delay time during which a needed replacement item is being obtained.
• Administrative Time – That element of delay time not included in supply delay time.
• Corrective Maintenance Time – That part of the maintenance time during which corrective maintenance is performed on an item.
• Preventive Maintenance TIme – That part of the maintenance time during which preventive maintenance is performed on an item.
• Not Operating Time – That element of uptime during which an item is not required to operate.
• Alert Time – That element of uptime during which an item is assumed to be in specified operating conditions, and is awaiting a command to perform its intended mission.
• Reaction Time – That element of uptime needed to initiate a mission, measured from the time command is received.
• Mission TIme – That element of uptime during which an item is required to perform a stated mission profile.

Concepts of Time (MIL-STD-721)

References:

1. MIL-HDBK-338, Electronic Reliability Design Handbook, 15 Oct 84
2. MIL-STD-721, Definitions of Terms for Reliability and Maintainability, 23 Oct 91