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. The status is changed to KIA (Killed In Action) if the death is later confirmed.
SG-9 was declared MIA on the planet Avnil in 1997. (SG1: "The First Commandment")
In 2000, 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")