Here's what 'unmasking' - the phrase everyone keeps using about Susan Rice - means
In the aftermath of accusations that former national security advisor Susan Rice tried to "unmask" the identities of members of the Trump transition team, the concept of "unmasking" has been all over the news.
"Unmasking" is a term used by the intelligence community that means revealing the identity of someone on a monitored communication.
The intelligence community frequently monitors the correspondence of foreign actors - such as businessmen, or ambassadors - operating on US soil. If the monitored foreign actor is speaking with a US citizen, the US citizen's name will be "masked" on the report or transcript to protect his or her identity.
The report or transcript would list the "masked" person with some kind of generic descriptor like "Person 1." In this way, Person 1's correspondence would have been "incidentally collected," meaning he or she wasn't targeted but some of their communications were collected because the person that he or she was speaking to was targeted. "Incidental collection" is reportedly how Trump officials' communications were collected during the transition.
The foreign actor being monitored does not need to be the subject of an investigation to be surveilled, nor does Person 1. However, if a member of the intelligence community decides they need more information on the intelligence reports he or she is receiving about the foreign actor, that person can request an "unmask" of Person 1. This does not mean Person 1 has committed a crime or is the target of an investigation, although Person 1 could also be unmasked if it was discovered that he or she had committed a crime.
"Unmasking" occurs when intelligence officials need more context about the surveillance on a foreign actor to make sense of the information they've received.
In the case of former national security advisor Michael Flynn, the National Security Agency was surveilling Russian ambassador Sergey Kislyak. When they overheard him speaking to a "masked" individual about lifting Russian sanctions, the intelligence official (it is unknown who requested his unmasking or who leaked his name) would have likely wanted to know if Kislyak was speaking to a friend who had no connection to the topic, or if he was speaking to someone like Flynn, with a political position. The intelligence officials therefore needed Flynn's name "unmasked" to decide whether they should investigate that correspondence.
Unmasking itself is not illegal. However, leaking an unmasked name to anyone is illegal.
The Republicans have put pressure on the NSA and FBI to determine who leaked the identity of Michael Flynn, who resigned shortly after his name was leaked.