SGML (standard generalized markup language) is an international standard for the definition of markup languages; that is, it is a metalanguage. Markup consists of notations called tags that specify the function of a piece of text or how it is to be displayed.

SMS (Short Message Service) is a text messaging service component of most telephone, Internet, and mobile device systems. The service allows users to send and receive messages of up to 160 characters (when entirely alpha-numeric) to and from GSM mobiles.

SQL (Structured Query Language) is a standardized programming language that’s used to manage relational databases and perform various operations on the data in them. Initially created in the 1970s, SQL is regularly used not only by database administrators, but also by developers writing data integration scripts and data analysts looking to set up and run analytical queries.

Systems Network Architecture (SNA) is IBM’s proprietary networking architecture, created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes formats and protocols and is, in itself, not a piece of software. The implementation of SNA takes the form of various communications packages, most notably Virtual Telecommunications Access Method (VTAM), the mainframe software package for SNA communications.

The Domain Name System (DNS) is the Internet’s system for mapping alphabetic names to numeric Internet Protocol (IP) addresses like a phone book maps a person’s name to a phone number. For example, when a Web address (URL) is typed into a browser, a DNS query is made to learn an IP address of a Web server associated with that name.

A cathode-ray tube is a vacuum tube containing one or more electron guns, the beams of which are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms, pictures, radar targets, or other phenomena. A CRT on a television set is commonly called a picture tube.

A CD-ROM (compact disc read-only memory) is a pre-pressed optical compact disc that contains data. Computers can read but not write or erase CD-ROMs, i.e. it is a type of read-only memory.

A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions in the program.

A compact disc (CD) is a portable storage medium that can be used to record, store and play back audio, video and other data in digital form.

A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents.