Scientific advisor was the title given by UNIT to their personal adviser on science and technology-related matters.
In the 1970s, the British branch of UNIT originally selected Liz Shaw to be their scientific advisor, but this role soon went to the Third Doctor, with Liz staying on as his assistant. The Doctor served as UNIT's scientific advisor throughout his exile on Earth. (TV: Spearhead from Space onwards) By the time of Operation Blue Sky in 2009, the Doctor, by then in his tenth incarnation and having cut ties from UNIT shortly after his regeneration into the Fourth Doctor, was still technically working for UNIT, as he had never officially left. (TV: Robot, The Sontaran Stratagem, The Day of the Doctor)
By 2010, UNIT's scientific advisor was Malcolm Taylor. (TV: Planet of the Dead)
Later, the position was given commanding authority under the "science first" doctrine when Kate Stewart became Head of Scientific Research. (TV: The Power of Three)
Other occurrences edit
The Intrusion Countermeasures Group, UNIT's predecessor, had a "scientific advisor" role as well. Rachel Jensen originally filled it. (TV: Remembrance of the Daleks)
Joking, the Fourth Doctor offered Duggan a position as his scientific advisor. (TV: City of Death)