The Left Channel may refer to one of a pair of Channels in a Stereo configuration, which is generally used for music. It may also refer to a channel meant to be positioned to the left of the screen in a Surround Sound configuration. For Matrix Surround the Left Channel may be used to derive additional Center and Rear Channel signals, along with the Right Channel signal.
Mono audio for movies may either be delivered only to the Center Channel, or divided between the Left and Right Channels.
The Left Channel may be referred as simply Left, as in the Channels are Left, Right, Center, Left Surround, and Right Surround.
The image below shows the position of the Left Channel in a 5.1 Surround Sound configuration.