Some thoughts:
'key' may/could be used when something's important, in other words, e.g. results a company reports upon internally or externally at board level. An 'objective' can be looked at as something wanted to be achieved and there may be a time component added (to reaching that). A 'target' can be something we are aiming at/for.
In this example we could say the objective of zero accidents by end of period 'x'. To monitor progress, review and take action as appropriate, we could use an 'indicator', and call it a KPI. Each month we could collect organisationwide and report the KPI measurement; e.g. September 'Zero' accidents, October 'Ten' accidents reported.
Hence, there appears a 'Performance' measure in place. Let me (and readers) keep in mind however, what the others have also said and debated; And to emphasise how simple (not unimportant) metrics can be abused.
I hope that helps.