Preventive maintenance (PM) has the following meanings:
1). The care and servicing by personnel for the intended purpose of maintaining equipment in a satisfactory operating condition by providing a continuous, routine, systematic inspection; detection; and correction of incipient failures either before they occur or before they develop into major defects. 2). Maintenance, including tests, measurements, and parts replacement like-new condition that are subject to wear, performed specifically to prevent faults from occurring. 
Preventive maintenance can be described as maintenance of equipment or systems before fault occurs. It can be divided into two subgroups:
The main difference of subgroups is determination of maintenance time, or determination of moment when maintenance should be performed. While preventive maintenance is generally considered to be worthwhile, there are risks such as equipment failure or human error involved when performing preventive maintenance, just as in any maintenance operation. Preventive maintenance as scheduled overhaul or scheduled replacement provides two of the three proactive failure management policies available to the maintenance engineer. Common methods of determining what Preventive (or other) failure management policies should be applied are; Original Equipment Manufacturer (OEM) recommendations, requirements of codes and legislation within a jurisdiction, what an "expert" thinks ought to be done, or the maintenance that's already done to similar equipment, and most important measured values and performance indications.
To make it simple:
- Preventive maintenance is conducted to keep equipment working and/or extend the life of the equipment.
- Corrective maintenance, sometimes called "repair," is conducted to get equipment working again.
The primary goal of maintenance is to avoid or mitigate the consequences of failure of equipment. This may be by preventing the failure before it actually occurs which Planned Maintenance and Condition Based Maintenance help to achieve. It is designed to preserve and restore equipment reliability by replacing worn components before they actually fail. Preventive maintenance activities include partial or complete overhauls at specified periods, oil changes, lubrication and so on. In addition, workers can record equipment deterioration so they know to replace or repair worn parts before they cause system failure. The ideal preventive maintenance program would prevent all equipment failure before it occurs.
There is a controversy of sorts regarding the propriety of the usage “preventative.” The consensus of internet entries concerning the respective usages seems to indicate that “preventive” is the preferred term. .
- UMTSN ECRI Institute, Universal Technology Services Nomenclature Terms and Definitions, 2007, www.ecri.org
- Michael Quinion, PREVENTATIVE OR PREVENTIVE, World Wide Words.
- What term is more correct preventive or preventative?, Answers.com.
- OIT Style Guide: How should I write that word? An A to Z, Office of Information Technology.
- Bobby Joseph, Letter to the Editor: What's the good word—preventive or preventative?, International Journal of Epidemiology, Vol. 30, N. 6, p. 1498.]