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Wiktionary
soft key

a. (context film of lighting English) gentle highlighting; low-key. alt. (context electronics English) A button, located along a display device, which performs whatever function is shown near it on the display. n. (context electronics English) A button, located along a display device, which performs whatever function is shown near it on the display.

Wikipedia
Soft key

A softkey or soft key is a button flexibly programmable to invoke any of a number of functions rather than being associated with a single fixed function or a fixed set of functions. A softkey is often located alongside a display device of a portable device such as a cellular phone, where the button invokes a function described by the text at that moment shown adjacent to the button on the display. However, softkeys are also found away from the display device, for example on the sides of cellular phones, where they are typically programmed to invoke functions such as PTT, memo, or volume control. Softkeys are generally found on keyboards (e.g. the F keys), cellular phones, Automated Teller Machines, Primary Flight and Multi-Function Displays, although they are also found elsewhere. Hard key is the contrary term, which means a hard-coded key such as a number key pad or the Send/End key of a mobile phone. Depending on the development company, cellular phone softkeys are often called F1(Left), F2(Right).