A U-235 nucleus being hit with some other particle causes it to undergo fission and emit more particles. Those particles eventually hit other U-235 nuclei, which propagates the reaction. What is this type of reaction called?

Nuclear chain reaction

explanation:

A uranium atom absorbs a neutron and do fission into to new atoms releasing neutrons and large amount of binding energy . The binding energy is used for further propagation of reaction.This causes release of more uranium atoms which collide and do fission causing release of new uranium atoms hence the reaction propagates in the form of chain hence called as nuclear chain reaction.