MTTR is an abbreviation that has several different expansions, with greatly differing meanings. It is wise to spell out exactly what is meant by the use of this abbreviation, rather than assuming the reader will know which is being assumed. The M can stand for any of minimum, mean or maximum, and the R can stand for any of recovery, repair, respond, or restore. The most common, mean, is also subject to interpretation, as there are many different ways in which a mean can be calculated.

• Mean time to repair
  Mean time to repair
  Mean time to repair is a basic measure of the maintainability of repairable items. It represents the average time required to repair a failed component or device. Expressed mathematically, it is the total corrective maintenance time divided by the total number of corrective maintenance actions...
  
  
• Mean time to recovery/Mean time to restore
  Mean time to recovery
  Mean time to recovery is the average time that a device will take to recover from any failure. Examples of such devices range from self-resetting fuses , up to whole systems which have to be repaired or replaced.The MTTR would usually be part of a maintenance contract, where the user would pay...
  
  
• Mean time to respond
• Mean time to replace

In an engineering context with no explicit definition, the engineering figure of merit

Figure of merit

A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application...

, mean time to repair

Mean time to repair

Mean time to repair is a basic measure of the maintainability of repairable items. It represents the average time required to repair a failed component or device. Expressed mathematically, it is the total corrective maintenance time divided by the total number of corrective maintenance actions...

 would be the most probable intent by virtue of seniority of usage. It is also similar in meaning to the others above (more in the case of recovery, less in the case of respond, the latter being more properly styled mean "response time

Response time

In technology, response time is the time a system or functional unit takes to react to a given input.- Data processing :In data processing, the response time perceived by the end user is the interval between the instant at which an operator at a terminal enters a request for a response from a...

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