From the Comprehensive Dictionary of the Teyvat Common Tongue

noun: catalyst; plural noun: catalysts

• a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
  "Hydro acts as a catalyst promoting the vaporization of Pyro and the amplification of elemental techniques"

• a person or thing that precipitates an event.
  "the Snezhnayan diplomat’s arrival acted as a catalyst for the Osial incident"

• a weapon type that allows a Vision user to perform elemental attacks and amplify their elemental abilities
  “the power of his giant Hydro whale was increased by 150% when wielding a catalyst”