A signal is a notification to a process that an event occurred. Signals are sometimes described as software interrupts. Signals are analogous to hardware interrupts in that they interrupt the normal flow of execution of a program; in most cases, it is not possible to predict exactly when a signal will arrive. They are defined in the C standards and extended in POSIX, but many other programming languages/systems provide access to them as well.
Standards
These standards actually place requirements on the signal-handling facilities:
ISO C99: section 7.14 "Signal handling <signal.h>" in draft n1256.
ISO C11: section 7.14 "Signal handling <signal.h>" in draft n1570.
POSIX-2008: section "signal.h - signals" in IEEE Std 1003.1-2008 (aka The Open Group Base Specifications Issue 7).
C++11 essentially says that you get the same facilities as in C, as long as you restrict signal handlers to the common subset of C and C++, and use C linkage for them. Quoting n3242 section 18.10 "Other runtime support" [support.runtime] (paragraph 8),
The common subset of the C and C++ languages consists of all declarations, definitions, and expressions that may appear in a well formed C++ program and also in a conforming C program. A POF (“plain old function”) is a function that uses only features from this common subset, and that does not directly or indirectly use any function that is not a POF, except that it may use functions defined in Clause 29 that are not member functions. All signal handlers shall have C linkage. A POF that could be used as a signal handler in a conforming C program does not produce undefined behavior when used as a signal handler in a C++ program. The behavior of any other function used as a signal handler in a C++ program is implementation-defined.
