The following is an example of a Federal regulation defining the term waiver of Preemption.
According to 49 CFR 105.5 [Title 49. Transportation Subtitle B Other Regulations Relating To Transportation Chapter I Pipeline And Hazardous Materials Safety Administration, Department Of Transportation Subchapter A Hazardous Materials And Oil Transportation Part 105 Hazardous Materials Program Definitions And General Procedures Subpart A Definitions] Waiver of Preemption means “a decision by the Associate Administrator to forego preemption of a non-Federal requirement -- that is, to allow a State, political subdivision or Indian tribe requirement to remain in effect. The non-Federal requirement must provide at least as much public protection as the Federal hazardous materials transportation law and the regulations issued under Federal hazardous materials transportation law, and may not unreasonably burden commerce.”
Preemption is the rule of law that if the federal government through Congress has enacted legislation on a subject matter it shall be controlling over state laws and/or preclude the state from enacting laws on the same subject if Congress has specifically declared it has "occupied the field."