A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place.

The neutron-fission chain reaction: a neutron plus a fissionable atom causes a fission resulting in a larger number of neutrons than was consumed in the initial reaction.
Chemical reactions, where a product of a reaction is itself a reactive particle which can cause more similar reactions. For example, every step of H₂ + Cl₂ chain reaction consumes one molecule of H₂ or Cl₂, one free radical H· or Cl· producing one HCl molecule and another free radical.
Electron avalanche process: Collisions of free electrons in a strong electric field forming "new" electrons to undergo the same process in successive cycles.