I have trouble knowing what to call this because it seems fairly uncommon, hence the question. Once I find the correct vocabulary I can update the question title.
I am looking for the best type of audio-listening equipment, for either one ear or two ears, which makes it the most difficult for others in proximity to the wearer to hear what the wearer is listening to. Basically, noise canceling in reverse. The use case for this is for a developer to listen to music etc in an office with many others which is usually very quiet, without distracting other workers with small background noises. I like to be polite to others working conditions.
For the purposes of this question I am not concerned with price, wireless capability, or superb audio quality.