Chinese term (edited): 自环头

一般称为“自环头”。

http://sqlserver.blog.51cto.com/149783/80462
自环头的作用：
单机既是发送端又是接收端，既是服务器端又是客户端，通常用于程序测试。
制作方法：
取一段10厘米左右长的网线，将外皮剥掉，随便拿其中的一对，插入水晶头中，线序为：
1连接3
2连接6
然后用网线钳压紧。

http://www.h3c.com.cn/Service/Document_Center/IP_Network_Pro...
external：进行外环测试。该测试需要在交换机端口上使用特制自环头（自环头能使端口发出的报文直接被端口接收。对于百兆电口使用的自环头，是用八芯网线中的四芯制作的，对于千兆电口使用的自环头，是用八芯网线中的八芯制作的），可定位该端口的硬件功能是否出现故障。
internal：进行内环测试。该测试在交换芯片内部建立自环，可定位芯片内与该端口相关的功能是否出现故障。

http://www.google.com/search?hl=en&q=define: loopback&btnG=G...

Definitions of loopback on the Web:

A method of checking the accuracy of data transmission in which the transmitted data stream is returned, or looped back, to its source for ...
www.cxrlarus.com/assets/glossary.html

Ability at the uplink to receive its own signal downlinked from the satellite for verification.
www.7e.com/glossary/l.asp

http://en.wikipedia.org/wiki/Loopback
The term loopback (sometimes spelled 'loop-back') is generally used to describe methods or procedures of routing electronic signals, digital data streams, or other flows of items, from their originating facility quickly back to the same source entity without intentional processing or modification. This is primarily intended as a means of testing the transmission or transportation infrastructure.

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Note added at 1 hr (2008-07-14 08:43:48 GMT)
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Telecommunications
In telecommunications, loopback, (short loop) is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems.As a test, many data communication devices can be configured to send specific patterns (such as all ones) on an interface, and can detect the reception of this signal on the same port. This is called a loopback test and can be performed within a modem or transceiver by connecting its output to its own input. A circuit between two points in different locations may be tested by applying a test signal on the circuit in one location, and having the network device at the other location send a signal back through the circuit. If this device receives its own signal back, this proves that the circuit is functioning.

A hardware loop is a simple device that hardwires the receive channel to the transmit channel. In the case of a network termination connector such as X.21, this is typically done by simply connecting the pins together in the connector. Media such as optical fiber or coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.

A modem can be configured to loop incoming signals from either the remote modem or the local terminal. This is referred to as loopback or software loop.