(1) A substance that speeds up a chemical reaction or lets it take place under different conditions. (2) Someone or something that brings about or speeds significant change or action.
Example:
The assassination of Archduke Ferdinand in Saravejo in 1914 acted as the catalyst for World War I.
Although the Great Depression was a difficult and tragic period in this country, it served as the catalyst for many important social reforms. The Social Security Act of 1935 helped provide security for retired workers; it in turn became the catalyst for a number of laws concerning disabled and unemployed workers, health insurance, on-the-job safety, and dependents of deceased workers. The Depression was also the catalyst of many public-works projects, which were designed to put the unemployed back to work.