Though Terrance Dicks' novelisations call it the Chameleon Circuit as far back as 1975. One of many details that start out in the Target books and later get confirmed onscreen having become fanon in the meantime.
That's interesting. What are some of the others?
Of course, since Dicks was the show's story editor for some time, these could be the kind of details that are part of the series bible or behind-the-scenes materials but only belatedly get mentioned onscreen.
Ooh, you've caught me there. The inevitable problem is that having read and re-read those early novelisations in the days before I had a video, I tend to forget what's actually come from the books! Even though I've now seen them four or five times, I still get taken by surprise when the actual episodes don't include lines or scenes that were in the book...
Pretty sure though that the Brigadier's middle name, Gordon, first turned up in the books. And a lot of the stuff about the Doctor's academy-era relationships with the Master and Borusa come more from Barry Letts and Malcolm Hulke's novelisations of the Delgado stories, and Terrance's novelisation of The Deadly Assassin (which would have just gone across Graham Williams and Anthony Read's desks for approval when they were rush-writing Invasion of Time) than the actual episodes.
But to be strictly accurate, I just pulled a couple of the old books off the shelf, and Terrance doesn't actually use the words 'chameleon circuit' in Doctor Who and the Terror of the Autons (published March 75). Instead, we get "The Doctor knew that the Master's TARDIS, unlike his own, still had its chameleon mechanism in working order." And then a bit later, "...on one of his visits to twentieth century London, the chameleon circuits had worn out, and he had been unable to replace them." (So close, but not quite there. Six pages later, the Doctor recognises a bomb as "a Sontaran fragmentation grenade" and he didn't say that in the original story!)
Checking back a bit, Malcolm Hulke's April 1974 novelisation of Colony in Space, The Doomsday Weapon, has a Time Lord observe that one of the two defects to the Doctor's TARDIS is "that particular TARDIS had lost its chameleon-like quality. It was in for repairs..." Seeing as Hulke wrote that Cushing radio pilot, and he was Terrance Dicks' best friend and sometime landlord, it seems a fair bet that he actually came up with the phrase (in the same chapter, he also comes up with the title Keeper for the guardian of the Time Lords' files, though it's Keeper of the Files, not 'of the Matrix').