Military acronym - OPerational SECurity. Most familiar when summed up by the WWII American propaganda "loose lips sink ships," OPSEC is the practice of masking one's intent by keeping secrets. In wartime, the destinations and manifests of cargo transports are kept secret, because from those seemingly harmless pieces of information, troop strength and locations can be inferred. If an enemy operative learns that cargo planes are departing once a week with 420,000 MRE's, then it's trivial math to deduce that somewhere near that plane's destination there are 20,000 people who require food.
In a society with a free press, OPSEC is even more difficult to practice. Because news agencies compete to make the most information public, there is an evolutionary pressure on news organizations to find out things that are supposed to be secret. At the first hint of an air war, local TV stations set up cameras just outside the fences at air force bases and take footage of our aircraft taking off. During Desert Storm, CNN showed planes leaving Saudi Arabia live on TV, allowing anyone with a copy of Jane's to calculate when they'd hit Baghdad. Anyone with an interest in shooting down those aircraft would have advance warning--certainly not the intent of the news crew!
Those are the easy examples of OPSEC--you ask the news crew to be more subtle in their reporting, or to delay broadcast of certain images. The hard part of OPSEC is cleaning up the fingerprints you didn't know you were leaving. The first people outside the Pentagon to know that we had entered a shooting war with Iraq were not soldiers. They were not reporters. They were Domino's Pizza managers in the Washington, D.C. area, who got calls from several branches at once, saying that there had been a massive run on deliveries to the Pentagon and that they needed more staff to help out.