MIA is a common military term for when a soldier or member of a team goes Missing In Action. If someone is declared Missing In Action it means that they went missing, often, but not always, captured by the enemy, whilst on a mission for a Government or, more often, a Military. MIA also generally indicates that the armed force in question has no active rescue operations currently underway concerning these parties.
SG-9 was declared MIA on the planet Avnil in 1997. (SG1: "The First Commandment")
Colonel Jack O'Neill was declared MIA by Major General George S. Hammond when he was trapped on Edora. (SG1: "A Hundred Days")
In 2009, Jeremy Franklin was declared MIA when he disappeared from Destiny's chair interface room. (SGU: "Pain")