A warning is often issued on recognizing a potential high-risk situation, a probable misunderstanding, degraded service or imminent failure.
A warning-level error message shown by a compiler indicates a piece of code which the compiler recognizes as being potentially a problem, but which is syntactically correct such that the compiler is able to continue.
The seriousness of warning may vary from basically an error to something that could be considered just as bad programming style. Most of compilers provide keys to disable default warnings and may provide keys to enable warnings that are disabled by default (e.g. -Wall, -Wextra, -Werror).
