Clarke's Law
Clarke's Law stated that:
Any sufficiently advanced form of technology is indistinguishable from magic.
The Captain paraphrased this by describing now-wrecked craft had possessed technologies "indistinguishable from magic." (TV: The Pirate Planet)
The Seventh Doctor reminded Ace of Clarke's Law and stated that "the reverse is true", as was the case of the Thirteen Worlds, a parallel universe where technology formed alongside magic. (TV: Battlefield)
When Angelo Colasanto believed Captain Jack Harkness' Vortex manipulator to be magic, Jack corrected him saying it was technology, although the two could be indistinguishable. (TV: Immortal Sins)
The Twelfth Doctor paraphrased the law to Clara Oswald when stating how he would impress a Viking village with a Yo-yo. (TV: The Girl Who Died)
Behind the scenes
In the real world, this is actually Arthur C. Clarke's Third Law.
