Call holding is a phone feature that allows the user to put an ongoing call on hold, while at the same time making or receiving a second call on the same phone.
The person 'on hold' will not be aware of what the other party is doing during the call holding period, and they may be presented with silence or hear pre-recorded music and reassuring messages. This feature is convenient when a user needs to talk with two parties who should not talk directly (i.e. a conference call is not possible), which might arise when a mortgage broker is setting up a deal between a bank and a mortgager.
Call holding can also be used in conjunction with call waiting to allow two calls to be handled at once, so ensuring that important incoming calls are not missed.