What is a trigger in CRM?

Let’s say a prospective customer clicks a link inside an email, this can be a trigger for changing his status from 'cold lead' to 'warm lead'. Or when a prospective customer fills out a website form, this action can trigger a series of marketing emails. Or when a customer pays an invoice, this action can trigger a task creation for an account manager. These are the examples of real-world CRM sales and marketing triggers that are most popular.
Trigger-based marketing software allows companies to significantly save on labor costs, avoid human errors, and improve customer experience. The term 'event based marketing' is closely related.