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 recovery/Mean time to restore
- Mean time to respond
- Mean time to replace
In an engineering context with no explicit definition, the engineering figure of merit, mean time to repair 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").