C (pronounced see, like the letter C) is a general-purpose programming language initially developed by Dennis Ritchie between 1969 and 1973 at Bell Labs. Its design provides constructs that map efficiently to typical machine instructions, and therefore it found lasting use in applications that had formerly been coded in assembly language—most notably system software like the Unix operating system. C is one of the most widely used programming languages of all time, and there are very few computer architectures for which a C compiler does not exist.
Before there was an official standard for C, many users and implementors relied on an informal specification published in a book by Ritchie and Brian Kernighan; that version is generally referred to as “K&R” C. In 1989 the American National Standards Institute published a standard for C, generally called “ANSI C” or “C89,” and the next year the same specification was approved by the International Organization for Standardization; this is generally called “C90.” ISO released an extension to the internationalization support of the standard in 1995, and a revised standard (known as “C99”) in 1999. The current version of the standard (now known as “C11”) was approved in December of 2011.