The Osterhagen Key was a device that was held by UNIT by the time of the New Dalek Empire's invasion of Earth.
After the planet fell to the Daleks, UNIT gave the key to Martha Jones who was to use Project Indigo to escape. Former Prime Minister Harriet Jones told Martha Jones to never use the key. (DW: Stolen Earth)
Word root possibly means "Eastern City" or "Fossil City", depending on spelling.
"Osterhagen" is an anagram of "Earths Gone"
speculation : http://www.leytonjay.co.uk/sci-fi/what-is-the-osterhagen-key
