"Unmasking" is the term being used in the press about decisions to uncover people who were incidentally caught up in routine surveillance of foreign officials, which reportedly happened to members of President Trump's transition team.
Here's how to think about the procedure that's being described, what unmasking can lead to, and why it matters.
U.S. intelligence agencies routinely do legal surveillance of foreign nationals thanks to a section of law in the "Foreign Intelligence Surveillance Act," generally referred to as FISA.
FISA only authorizes electronic surveillance of non-U.S. people, but it can pick up communications in which a U.S. citizen is one of the parties in the conversation. For example, if the ambassador another country under surveillance calls a U.S. citizen, both would be captured.
In that example, when the phone call or other communication is eventually transcribed into an intelligence product, the name of the U.S. citizen is not supposed to be identified in the document, to stay consistent with FISA. Instead, the intelligence agencies use a simple alias, such as "U.S. person number 1," rather his or her proper name. This is known as masking, and masking should also apply in cases where two foreign nationals simply mention the name of a U.S. citizen.
However, there are times when the intelligence agency needs to know the name of the person in the communication, and so they must ask that the alias be removed. Removing the alias is known as "unmasking."
There are legal reasons to unmask someone, and officials are required to follow proper procedures to make sure the unmasking is being done for a good reason. But it's also possible that someone could have been unmasked illegally, and that's what Trump's team says happened.
Specifically, Trump's team charges that the Obama administration unmasked some of the Trump transition team officials for political reasons, in order to show that Trump's team was talking to the Russians.
A webpage from the Office of the Director of National Intelligence calls the process of keeping unnecessary information about U.S. citizens out of intelligence reports "minimization." The page says these efforts "minimize the acquisition, retention and dissemination of incidentally acquired information about U.S. persons."
In addition to having procedures meant to keep private the names of U.S. people incidentally caught up in foreign surveillance, the law says the amount of time that the document is kept should be low, and sharing the document between agents and agencies should also be kept low.