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 H 2 + Cl 2 chain reaction consumes one molecule of H 2 or Cl 2 , 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.
• In linguistics, language change is seen as the gradual outcome of chain reactions and subject to cyclic drift.

"Chain Reaction" was a hit single for Diana Ross that was written by the Bee Gees, who also provided the backing vocals for the single. The track was released on the album "Eaten Alive" in 1985 and as a single the following year.

According to the Gibbs biography, the brothers had initial reservations about offering the song to Diana in case it was too Motown for her.

The single became her first #1 single in the UK since I'm Still Waiting in 1971. The song also reached #1 in Australia and was the top selling single of the year (1986) in that country. The song fared less well in the USA where it peaked at #66 on the Billboard Hot 100. A Hi-NRG remix by François Kevorkian and Ron St. Germain was released on 12".

In 2001, British pop group STEPS covered "Chain Reaction". The song reached #2 in the UK. The song was performed by the Top 4 Finalists on American Idol (season 6).

For the Diana Ross original, see Chain Reaction (song). "Chain Reaction" is a pop song for British group Steps, for their greatest hits album Gold - The Greatest Hits. The song was originally sung by former Supremes singer Diana Ross. She sang it in 1986 and her version reached #1 in Australia. The Steps version made it to #2 in the UK.