Line-in recording is a term often used by manufacturers of sound equipment to refer to the capability of a device to record line level audio feeds.
Microphone and instrument inputs, by contrast, are designed for devices which require further amplification to be at line-level.
The common 3.5 mm line-in connector has the left channel on the tip and right channel in the middle.
[1] Line-in is most commonly used for instruments.
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