I’ve notice sterile is a term used especially in FAA which means “only only activities required for the safe operation of the aircraft may be carried out by the flight crew, and all non-essential activities in the cockpit are forbidden” and I look it up in cambridge but its meaning doesn’t really have anything to do with aviation even when I look it up in cambridge dictionary, so why would it be used? It’s just something I brought up, I was curious myself that’s all.
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Good afternoon, and good question!
Plenty of aviation words have different meanings outside of aviation, and those are often the more widely known versions you’ll find in dictionaries. Like, “sterile” in most senses refers to either being unable to reproduce, or being devoid of life. But in aviation it just means to be focused on procedural topics.
Another example would be the word “final.” It can mean a ton of things in a ton of different contexts. Final can just be the adjective for the last in a series. It can also mean the closing game in a tournament. In aviation, it’s the last segment of a flight when the plane is on the approach and about to touch down. That definition isn’t listed in most dictionaries, but it’s legitimate nonetheless. There are many, many other examples, but I hope this helps!
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Adding on to @Poxy, the dictionary definition of “sterile” can definitely be applied to aviation. There’s two definitions of sterile recognized by Oxford - “not able to produce children or young” and “free from bacteria or other living microorganisms; totally clean.”
The core idea of a “sterile cockpit” is that it’s devoid of any distractions or non-essential conversation, so it is “totally clean” in a way.
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Adding on to what @Mort and @Poxy said; US Federal regulations require all cockpits to be sterile until reaching 10,000 feet agl after departure, and these rules are reinstated upon descending through 10,000 on arrival.
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Lots of great answers here but I’d still love to chime in from other angles 
When the pilots switches on Sterile Light (and yes, there is such a lamp 
in some airlines) the feel/ intuition is like this:
Sterile means no germs no bacteria, so in this context it means No Contamination.
For example upon Landing.
When that sterile light is on, the work environment in the cockpit must now be devoid of contamination from OTHER non-useful items or conversations unrelated to the landing procedures. A blue light outside the cockpit door also indicates to the cabin crews to not bother the pilots at this time (unless there are critical emergencies the pilots needed to know).
This means, during a sterile cockpit moment: The minds of the pilots must be clear, think almost as if the pilots need to meditate. No joking, no distractions, no talking about other things, no other people allowed in, no contamination from other subjects! The pilots must strictly FOCUS on trying to perform whatever procedure is going on at that time… especially when you are closer to the ground (i.e. under 10,000 feet), closer to the ground means you only have very little time to fix any mistakes that could happen.
Very sterile, isn’t it?
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In simple terms, minimal communication and distractions. It’s often used in the most complex of operations during a routine flight, like taxiing on an busy airport, takeoff, landing, or in an emergency
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Tragically, one of the two accidents specifically cited by the FAA in establishing the sterile cockpit rule was the crash of Eastern Air Lines Flight 212, which claimed the lives of Stephen Colbert’s father and two brothers. The aircraft was on an ILS approach in dense fog when the crew’s non-essential conversation—including topics like politics and used cars—contributed to a loss of situational awareness, as confirmed by the NTSB investigation.
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It’s interesting how many of the terms used in aviation are widely known in other definitions. I wouldn’t have know why aviation would also use these terms especially like sterile, I’m guessing similar meaning perhaps