Metaphorical extension
Encyclopedia
A metaphorical extension is the "extension of meaning in a new direction" through popular adoption of an original metaphorical comparison.

Metaphorical extension is almost a universal and natural process in any language undergone by every word. In general, it's not even perceived in everyday usage as meaning change. When its least obvious, users don't even see it as extending the meaning of a word. Consider the example of illuminate: it originally meant "to light up" something dim or dark, but has evolved to mean "to clarify", "to edify". After a while these new meanings seem so natural as to be integral parts of the word, where senses such as "to celebrate" and "to adorn a page with designs" seem like more obvious additions.

Radiation

According to linguist Jeffrey Henning:
Radiation is metaphorical extension on a grander scale, with new meanings radiating from a central semantic core to embrace many related ideas. The word head originally referred to that part of the human body above the rest. Since the top of a nail, pin or screw is, like the human head, the top of a slim outline, that sense has become included in the meaning of head. Since the bulb of a cabbage or lettuce is round like the human head, that sense has become included in the meaning of head. Know where I'm headed with this? The meaning of the word head has radiated out to include the head of a coin (the side picturing the human head), the head of the list (the top item in the list), the head of a table, the head of the family, a head of cattle, $50 a head.


Other words that have similarly radiated meanings outward from a central core include the words fire, root and sun.

Examples

A crane at a construction site was given its name by comparison to the long-necked bird of the same name. When the meaning of the word daughter was first extended from that of "one's female child" to "a female descendant" (as in daughter of Eve), the listener might not have even noticed that the meaning had been extended.

The late Admiral, mathematician, and computer pioneer, Grace Murray Hopper, frequently repeated a favorite amusing story in front of many audiences about an early computer that experienced an episode where it kept calculating incorrectly. When technicians examined the machine's hardwired logic (the wiring, in the World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 era computer), a huge moth was discovered such that its body was shorting out one of the vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 and relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...

 circuits and causing the repetitive fault, and so may have played a key role in popularizing the term which was in use in hardware engineering from at least Edison
Thomas Edison
Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

's time (see Computer bug#Etymology). Since the Admiral's speeches, both hardware faults and software errors are now routinely referred to as "bugs", and getting the flaws out of a product is known as 'debugging
Debugging
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...

'
the system. Her team almost certainly coined the latter term. A particular type of software program designed to aid software development is known as a "debugger
Debugger
A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...

" compliments the program known as a compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...

, the first of which was developed by G.M. Hopper.

The use of bug to refer to a computer error in logic was a metaphorical extension that has become so popular that it is now part of the regular meaning of "bug". The computer industry has a host of words whose meaning has been extended through such metaphors, including "mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

" for possessing a 'tail' similar to said rodent now used widely for these computer input devices—even though ironically, the more modern wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...

one's have lost the metaphorical tail entirely.
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