With an Intermediate switch, or an intermediate switch, you can operate a lamp between 2 switches.
To give an example: if you have a switch above and below to operate 1 lamp, but you have a staircase with a mezzanine, you can operate the same lamp with an intermediate switch.
These 3-way intermediate switches are only available with 1 toggle switch and can be connected 3-way.
These 3-way intermediate switches are not available in a dimmer version and cannot be combined with dimmer switches. So can only be combined with toggle switches.