Speech code is spoken language that seems to mean one thing but really means another. I first saw the term used with this meaning in the "Quiller" novels of one of my favorite spy fiction authors, Adam Hall. Two spies might use speech code when, for example, they speak on a telephone line that may be tapped. One spy might say "the train arrives at midnight" to mean "the ambassador will leave the embassy compound at noon."
Speech code has two purposes:
Sometimes, speech code is arranged in advance. Key phrases are memorized so that each party can know what the other means. In this case, the real meaning of a sentence can be completely different than its surface meaning. For example, "it's snowing here" might mean "please ask Q to send the portable helicopter."
Under circumstances which are more fluid, the two parties may make up the code as they go along, each relying on the other's knowledge of the context of the conversation to provide the necessary translation. For example, a spy working in an enemy's aircraft factory might tell her control "I went hiking and took some pictures of an eagle today" when she meant "I managed to photograph the fighter plane."
I like the whole idea, so I chose my Internet domain, speechcode.com, based on it. Who knows? Perhaps this entire web site is in code.