Questions

There are many possibilities for this.

We can hold a breathe, meeting, conversation, party, grudge, etc... but cannot touch them.

Every computer on the internet needs a string called Internet Protocol (IP address) to perform its work. So Every computer on the internet is known as Host.

Any of the basic mechanical devices for applying a force are called simple machines.

There are 6 basic simple machines; the lever, the wheel and axle, the inclined plane, the wedge, the pulley, and the screw. Several of these simple machines are related to each other. But, each has a specific purpose in the world of doing work.

One word substitute for "That which cannot be heard" is Inaudible.