Several or many

A series of things or instances

एक श्रृंखला

English Usage: He collected multiple books on the subject.

Hindi Usage: उसने इस विषय पर एक श्रृंखला की किताबें इकट्ठा कीं।

A substance that increases the rate of a chemical reaction

Something that precipitates an event or change

कुछ जो एक घटना या परिवर्तन को तेज करता है

English Usage: Her speech was a catalyst for change in policy.

Hindi Usage: उसका भाषण नीति में परिवर्तन के लिए एक उत्प्रेरक था।