Sidetone is audible feedback to someone who is speaking. The term is most used in the telecommunication field.
In telephony, sidetone is the effect of sound picked up by the telephone's transmitter (mouthpiece) and instantly introduced at a low electronic signal level into the receiver (earpiece) of the same handset, a form of feedback.
Usability studies by Research In Motion (manufacturers of BlackBerry smartphones), LG and Motorola have demonstrated that a lack of sidetone has a tendency to make the user of a phone or cellular handset characterize it as dead or disconnected. In the same battery of tests, it was found that the presence of sidetone prevents users from needing to examine the device's display to determine if a call is still active. First introduced in the StarTAC handset, almost all cellular handsets manufactured by Motorola have sidetone, though its level of feedback adjustable by the user. Too much sidetone causes users to hear their own voice loudly which is why it is not standard on all cellular handsets and leaves the decision to incorporate sidetone up to the manufacturers. In usability studies prior to the launch of Apple's first-generation iPhone, users were quoted as feeling uncomfortable when the amount of sidetone is too high and will lower the level of their voice unnecessarily. Currently, iPhones (3G and later) and almost all Android, Palm, and Windows mobile devices incorporate some form of sidetone.
Sidetone in 19th century telephones varied until the carbon transmitter was used, which produced a distinct sidetone that discouraged speaking loudly enough, and occasionally so loud as to cause the instrument to produce uncontrolled oscillations, resulting in howling audio effects. Sidetone is disabled when phones of any kind are running in speakerphone mode, due to perpetual and almost immediate feedback. Anti-sidetone circuitry incorporating the principle of the hybrid coil brought sidetone under control in the early 20th century, leaving enough feedback signal to assure the user that the phone is really working, and allowing the use of a unitized telephone handset. In cellular technologies, one of the many benefits of sidetone-enabled phones is that a user knows a call has been dropped or ended if he or she no longer hears sidetone.
Digital telephones lack the mechanical acoustics and circuitry that used physical wiring to produce sidetone in older landline phones, so digital phones include electronic circuitry, software and firmware to reproduce sidetone. Many cell phones do not provide adequate sidetone despite general agreement among leading industrial design & usability experts who claim it is an important feature in cell phones, perhaps even more so than for land-lines because of the less predictable acoustics one will encounter while using a cellular phone.
Almost all land-line (wired and wireless) phones have employed sidetone, so naturally it was an expected convention for cellular telephony but is not standard by any means. Usability experts believe that lack of adequate sidetone causes some people to shout or speak too loudly when using a cell phone (this behavior is often referred to as "cell yell").
Sidetone is valuable for the hearing impaired. The amount of sidetone typically found on land-lines is 8%, and is 4% for cellular phones. Sidetone can be, and often is, amplified for land-line phones for the hearing impaired. In VOIP technologies such as Skype, sidetone has been experimented with but has not been formally adopted by software or hardware & accessories creators. Several software packages and wiring workarounds have been developed that replicate sidetone, but feedback looping remains a problem.
In wireless telegraphy (WT) and amateur radio, sidetone is the audible indication of a continuous wave (CW) signal as the operator sends Morse Code. As in telephony, sidetone serves as feedback to the operator that what they are sending is what is intended.
It is designed to mimic the tone generated by a typical radio receiver when a CW signal is converted to the intermediate frequency (IF), then mixed with the Beat frequency oscillator (BFO) frequency to generate a difference frequency, which is audible over the radio receiver loudspeaker or headphones.
Sidetone is also used on voice radio equipment to give the radio operator confidence that they are transmitting over the radio. The sidetone audio is typically derived from the transmit audio circuitry.
Public address systems 
When a commentator, announcer, or MC for a public event may otherwise be able to hear their own voice in the delayed output from the loudspeakers, they may opt to use a headset which provides instant sidetone of their own voice, thus removing the distracting effect of greatly delayed feedback from the loudspeakers.