"Those activities involving continuous or periodic monitoring and diagnosis in order to forecast component degradations so that as-needed, planned maintenance can be performed prior to equipment failure."
Monitoring means watching carefully
Diagnosis means to determine health status
Forecast means to project or predict future status
Degradations means deficiencies (relative to best possible health)
Maintenance in the context of the definition means to return the health to normal
Prior to failure means before complete loss of function or incapacitation
So the philosophy of Predictive Maintenance is to: Watch carefully, determine health status (on a continuous or periodic basis), predict future status based on deficiencies found and return health to normal before complete loss of function.