Aprioritron
On entering the War Master's TARDIS control room, the Eighth Doctor noted that the a prioritron, the randomiser and the safety locks were still operational. (AUDIO: The Castle of Kurnos 5)
Behind the scenes
In philosophy, something a priori can be known without sense experience, coming before experience, as contrasted with a posteriori knowledge, which is gained through interaction with the world. These terms are central to Immanuel Kant's pursuit of synthetic a priori knowledge, a priori knowledge which is not based on the meaning of the words in the statement. (e.g. "All bachelors are unmarried" would not qualify.)
Etymologically, the suffix -tron refers to an instrument or device, things one would typically use in the formation of a posteriori knowledge, rather than a priori.
External links
A Priori Justification and Knowledge at the Stanford Encyclopedia of Philosophy
The Analytic/Synthetic Distinction at the Stanford Encyclopedia of Philosophy