A computerised maintenance management system is what CMMS stands for, and the term is old enough that its spelling has drifted three ways: computerised with an s in British usage, computerized with a z in American, and computer maintenance management system where somebody dropped the -ised entirely. All three mean the same four things: a register of assets, a schedule against them, work orders raised on them, and the history those work orders accumulate. This page is about what the term covers, where its boundaries are, and why the acronym has outlived the technology that named it.
The name is a fossil and the function is not
The word computerised dates the term to when the alternative was a card index in the maintenance office. Nothing about the function is specific to computers now; what the term still usefully distinguishes is a system built round assets from a system built round jobs, tickets or projects.
The four things, in dependency order
The register first, because nothing else can exist without it. The schedule sits on the register. The work order records one job against one asset. The history is what accumulates, and it is the only thing that makes the first three worth maintaining honestly.
Where the term stops
It does not include spare-parts inventory, purchasing or capital planning, which are EAM. It does not include buildings-as-a-service management, which is facilities. It does not include condition-monitoring sensors. Products blur all three lines in marketing and the distinction decides what you should pay.
What it changes when it works
Not how well anybody maintains anything. It makes the register visible, the schedule staffable and the history retrievable, and those three let a maintenance manager argue with evidence rather than with conviction. On the worked example this site publishes, that argument is worth $111,283.20 a year in avoidable labour.
Questions people ask about computerised maintenance management system
Computerised or computerized?
Both, and neither is wrong. British spelling uses the s, American the z, and vendors index one or the other, so search both when comparing products.
Is computer maintenance management system a different thing?
No, it is the same term with a word dropped. It occasionally confuses people into thinking it means maintaining computers, which is IT asset management and a different subject entirely.
When did the term appear?
It became common with the first mainframe and minicomputer maintenance systems, which is why it carries a word nobody would choose now. The function predates the software by a long way.