This variable returns a list of the old version of sObject records. This list is only available in update and delete triggers.

trigger ApexTrigger on Opportunity (before update) 

    // Only available in Update and Delete Triggers

    for(Opportunity oldOpp: Trigger.old)

    { 

        for(Opportunity newOpp: Trigger.new)

        {

            if(oldOpp.id == newOpp.id && oldOpp.Amount != newOpp.Amount)

                newOpp.Amount.addError(‘Amount cannot be changed’); // Trigger Exception

        }

    }    

}

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