A call transfer is a telecommunications mechanism that enables a user to relocate an existing telephone call to another phone or attendant console, using a transfer button or a switchhook flash and dialing the required location.
This is typically done by putting the caller on hold and dialing the desired party/extension; they are then notified and, if they choose to accept the call, it is transferred over to them.
It is simply transferred to their line by way of a "transfer" key on the operator's phone or by keying in a string of digits which achieves the same function.
Other terms commonly used for an unannounced transfer include "unsupervised" and "blind".
[1] Unsupervised call transfer can be either warm or cold - depending on moment when B-leg is disconnected.