The concept of the "backdoor" in software has been around for a long time. When I started working with computers, a backdoor was usually something provided as a sort of emergency access shortcut for support people to use; a way of circumventing the usual access methods in order to fix something.
These days the term backdoor is more often used as a pejorative, implying a sneaky way for the government or hackers to break through program security and do nasty stuff behind the user's back.
The current bruhaha with Apple, over an iPhone possibly containing evidence of terrorist activities is far from the first time the concept of authorities requesting or demanding that backdoor access be provided on certain products or in certain cases.
AtlasObscuraCybersecurity experts have unanimously condemned the idea, pointing out that such backdoors would fundamentally undermine encryption and could exploited by criminals, among other issues. While a legal mandate or public agreement would be needed to allow evidence obtained via backdoors to be admissible in court, the NSA has long attempted—and occasionally succeeded—in placing backdoors for covert activities.