Shaft bow - When a gas turbine engine is shut down it will develop a circumferential thermal gradient vertically across the compressor due to hot air rising from the cooling metal components and pooling at the top. As the hot compressor rotor drum and casing cool and contract in the presence of this thermal gradient, they do so non-uniformly and therefore will bend slightly.
Article here on the subject.
https://www.researchgate.net/publica..._Initial_Study