A switch is a circuit that connects n inputs to n outputs by a one-to-one matching in between inputs and outputs.
For instance consider a switch for n = 2 (or a 2x2 switch). There are two inputs (In0, In1), and two outputs (Out0, Out1) of this switch. An additional select input (sel) determines the connectivity between inputs and outputs. If the select input is 0 (configuration 0), In0 is connected to Out0, and In1 is connected to Out1, whereas if the select input is 1 (configuration 1), In0 is connected to Out1, and In1 is connected to Out0.
Note that, at any given time, an input can only drive one output. Likewise, at any given time an output can only be driven by one input (i.e., unicast operation).