In many launch sequences I see the phrase 'engine' chilldown' coming up. What is this and why is it done?
Chilldown is needed for engines using cryogenic propellants -- liquid oxygen as the oxidizer most commonly, sometimes in conjunction with liquid hydrogen as the fuel. Pumping large amounts of very cold fluids into an engine resting at ambient temperature causes a variety of problems, so the plumbing and pumps need to be brought down to a working temperature before the engine starts. Chilldown is performed by allowing relatively small amounts of the cryogenic propellants through the system until it reaches a safely low temperature.
From Operation of a Cryogenic Rocket Engine by Wolfgang Kitsche:
A propulsion system running on cryogenic fluids has to pass a chill down phase before ignition. This chill down is indispensable, particularly if the propulsion system has turbo pumps to deliver the extremely cold fluids. The feed lines, turbo pumps, valves and ducts of the engine have to be brought down to the temperature of the cryogenic fluid. During the hot run the turbo pump delivers the fluid at high rotational speed. Starting the pump without prior chill down would cause cavitation [vapor bubbles produced in the stream], the bearings would not have sufficient cooling, and the fuel or oxidiser would be pumped into the chamber with an unacceptably high fraction of bubbles and gas.