TOMTIT was an acronym for Transmission Of Matter Through Interstitial Time, and was created at the Newton Institute by Stuart Hyde, Dr. Ruth Ingram and Professor Thascalos (an alias of the Master). TOMTIT could break down solid objects into light waves and transmit them from one place to another, similar to teleportation or a transmat, but utilizing interstitial time.
The Master altered TOMTIT to act as a simple time travel device (much like a time scoop) and pick up elements of Earth's military history to fight against UNIT when they attempted to remove him from his base of operations. Through repeated use, he summoned the Chronovore known as Kronos and an Atlantean named Krasis. (TV: The Time Monster)
It was eventually kept in the Black Archive, around the 2010s. Missy recognised it fondly after breaking in, and decided to "liberate" it, along with other weapons and souvenirs in the archive. (AUDIO: The Lumiat)