A judge's written authorisation to search a specified place for specified things - and the exceptions that let police search without one are broad.
A search warrant is an order signed by a judge or magistrate permitting a search. To obtain one, an officer submits a sworn written application - an affidavit - setting out the facts said to establish probable cause. The judge decides on that application, usually without anyone appearing for the person whose property is to be searched, which means the first opportunity to contest it normally comes long afterwards.
Two requirements are written into the Fourth Amendment itself and are the ones most often litigated. A warrant must be supported by oath or affirmation, and it must particularly describe the place to be searched and the things to be seized. That particularity requirement is the reason a warrant is not a general licence: a warrant authorising a search for a stolen vehicle does not authorise reading a diary, and a warrant for one apartment does not cover the building.
The more consequential practical point is that a great deal of lawful searching happens with no warrant at all. Consent, a search incident to a lawful arrest, items in plain view, certain vehicle searches, and genuine emergencies are all recognised bases for searching without one. Consent is the one a person actually controls: agreeing to a search waives the objection, and agreement given because someone believed refusal was not an option counts as consent unless it can later be shown otherwise.
Digital devices are treated differently from the containers they physically resemble. The Supreme Court has held that police generally must obtain a warrant before searching the contents of a mobile phone seized during an arrest, and separately that acquiring historical cell-site location records from a carrier is a Fourth Amendment search. A phone in a pocket is not, for this purpose, like a wallet.
Anyone shown a warrant is entitled to read it, and the two things worth reading are the address and the list of items - a search that goes materially beyond either is the kind of thing a lawyer can act on later, and only if someone noticed. Refusing consent to a search that is not covered by a warrant is not a crime and is not evidence of guilt, though physically obstructing a search may be a separate offence. Where a case depends on what a search produced, the affidavit behind the warrant is a document a defence lawyer will want early: warrants are challenged on what the officer told the judge, and that is only visible on the paper.
These directory pages each list at least one currently published law firm. The list is generated when this page loads, so a page whose listings have lapsed drops out rather than becoming a dead link.