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: