A price reduction for members of the military.
Sometimes this is used as a gimmick to entice military folks from a local base to shop at your location. A typical discount is 5% to 20%. Note that most places will not automatically give you the discount, even if you are in uniform. Many times you have to ask for it.
Another slang meaning we used to use in the US Navy was when you were using a purchase order to get things out of the military supply system. There's nothing like ordering parts that cost more than your house and car combined, and any E1 and up can pull it from the supply chain.
A less-used slang term is when a military unit takes over something from an enemy combatant, such as a small boghammer or a tank. The military discount was 100%, assuming it wasn't blown to bits.