What is a signal in Unix?

The manual refers to a very basic mechanism that allow processes or the operation system to notify other processes by sending a signal. The operation system can use it to notify programs about abortions of them (signal SIGABRT) or about a segmentation fault (often caused by accessing a null-pointer, SIGSEGV), to name two of them.

Some unix servers use signals so the administrator can use kill to send them a signal, causing them to re-read their configuration file, without requiring them to restart.

There are default actions taken for some signals and other signals are just ignored. For example on receive of a SIGSEGV, the program terminates, while receiving a SIGCHLD, meaning a child-process died, will by default result in nothing special.

There is a ANSI C standard function that installs a signal handler, which is a function that can execute some code when receiving a signal, called signal (read in man signal). In different unix's, that function behave different, so its usage is discouraged. Its manpage refers to the sigaction function (read man sigaction), which behaves consistent, and is also more powerful.

I cannot believe that people are not comparing things such as hardware and software or stressing OS at some points.

Comparison between a signal and an interrupt:

The difference is that while interrupts are sent to the operating system by the hardware, signals are sent to the process by the operating system, or by other processes. Note that signals have nothing to do with software interrupts, which are still sent by the hardware (the CPU itself, in this case). (source)

Definitions

1. process = a program in execution, according to the book below

Further reading

1. compare the signal to Interrupts and Exceptions

2. Tanenbaum's book Modern Operating Systems

A physical signal and a Unix signal are indeed different concepts. When a Unix signal is sent from one process to another, there is no specific corresponding physical signal. Unix signals are merely an abstraction so programmers can talk about processes communicating with one another.

Unix signals could have been called messages, events, notifications, or even a made-up term like "frobs". The designers just chose the name "signal", and it stuck.