Two meanings:
(a) The term is commonly used in the UK and Ireland to refer to historic building preservation, as in that “building has a preservation order”
(b) an order from a court to a party or potential party in a case, inquiry or dispute, or certain third parties to preserve evidence including documents and things relative to that case, inquiry or dispute. Under some procedural rules such an order can be granted even though no case has yet been filed. See Litigation Hold.