CAL

CAL (or Cal) has several different meanings relevant to Doctor Who.

In-universe
CAL is the computer core of The Library. Charlotte Abigail Lux (CAL) was the youngest daughter of the grandfather of Strackman Lux, who funded the expedition to find out what happened 100 years before. The Library was built to house Charlotte's mind and give her every book available to enjoy throughout eternity. When the Vashta Nerada attacked, CAL uploaded and saved the 4022 people in the Library at that time. (DW: Silence in the Library / Forest of the Dead )

Behind the scenes
CAL Video was a pioneer in the British CGI industry.

As name
"Cal" (or its variations) was the name of several characters:
 * Cal MacNannovich was a Platform One guest in DW: The End of the World and therefore also a comic character in the Doctor Who: Battles in Time comic strip's first story arc.
 * Chal was the leader of the Savages in the serial of the same name.