On-hook

In telephony, the term on-hook has the following meanings:

The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook. The weight of the receiver depresses the spring-loaded switchhook thereby disconnecting the idle instrument (except its bell) from the telephone line.
One of two possible signaling states, such as tone or no tone, or ground connection versus battery connection. Note: if on-hook pertains to one state, off-hook pertains to the other.
The idle state, i.e., an open loop of a subscriber line or PBX user loop.
An operating state of a telecommunication circuit in which transmission is disabled and a high impedance, or "open circuit", is presented to the link by the end instrument(s). Note: during the on-hook condition, the link is responsive to ringing signals.

The act of going on-hook is also referred to as releasing the line or channel, and may initiate the process of clearing. This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).