Common Abstract Tree Language. R. Voeller & Uwe Schmidt, U Kiel, Germany 1983. Universal intermediate language, used by Norsk Data in their family of compilers. "A Multi-Language Compiler System with Automatically Generated Codegenerators, U. Schmidt et al, SIGPLAN Notices 19(6):202-2121 (June 1984).
(From "catenate") Unix's command which copies one or more entire files to the screen or some other output sink without pause.
See also dd, BLT.
Among Unix fans, cat is considered an excellent example of user-interface design, because it delivers the file contents without such verbosity as spacing or headers between the files (the pr command can be used to do this), and because it does not require the files to consist of lines of text, but works with any sort of data.
Among Unix haters, cat is considered the canonical example of *bad* user-interface design, because of its woefully unobvious name. It is far more often used to blast a file to standard output than to concatenate files. The name "cat" for the former operation is just as unintuitive as, say, LISP's cdr.
Of such oppositions are holy wars made.