is a discipline of espionage
and military operations
that long predates the invention of telegraph
, which are the 'signals' allude
d to most often today. In fact, the term encompasses any
interception of another player's communication
Most inventions and advances in the fields of cryptography, steganography and good old-fashioned spying have come through the paired goals of intercepting and transmitting information that its originator very much wants to prevent anyone else from reading/hearing/interpreting/decoding/what-have-you. Some examples include the practice of tattooing messages on slaves' scalps, then allowing their hair to grow in before dispatching them on their mission (usually carrying a suitable impressive, encrypted and false missive in a more obvious form). The scytale was one of the first paired encryption/decryption devices, and was intended to counter SIGINT.
Signals, in other words, are any means of communication-at-a-distance that can be read. This includes, for example:
...and so on.
So. What, precisely, is SIGINT, then?
In sum, SIGINT is the trade and practice of gathering intelligence (information about another party's actions, disposition or intentions) from the signals sent/transmitted/dispatched by them. Ideally, this will involve gaining access to said signals in such a way that the sender is unaware that they have been read by anyone other than the intended recipient.
SIGINT is not always a purely passive activity. In one famous example, the United States Navy became aware, during World War II, that the Imperial Japanese Navy was gearing up to attack an American-held objective somewhere in the Pacific theater. One American codebreaker and analyst suspected the target was Wake Island. To test his hypothesis, the garrison on Wake was instructed (via hand-carried note) to report over the radio that their fresh water supply was in jeopardy due to mechanical failure. The codebreakers watched IJN traffic, and sure enough they were able to find a reference that the heretofore unnamed target was 'short of fresh water.' SIGINT (and some judicious deception) had resulted in an important bit of intel.
There are two complementary and equally important components of SIGINT. First, there is the traffic - the raw communications must be intercepted and delivered to those who can use it. This is the task of all manner of people, organizations and machines. Most (in)famously, the United States' National Security Agency and its associated chimaerical Echelon program, for example. The Rhyolite satellites. The 'coast watchers' - the American codebreaking unit mentioned above - are a famous example, as is Britain's MI6. Coastal radio stations around the world listen in on whatever they can pick up.
The second part of SIGINT is the analysis. This is usually a separate part of the process because in order to effectively decode, interpret and analyze an enemy's (or a friend's) signals, you must put a large amount of information, methodology and smarts in one place - and that place really can't be allowed to fall into anyone else's hands.