KPI is a key performance indicator. It's a quantitative measure of a company's performance over time to achieve a particular objective. There are different KPIs for different channels in digital marketing, below given are some KPIs for each channel
SEO : Impressions, Site Visits, Conversions, Revenue
Social Media : Followers, Engagement, Site Visits, Conversions
PPC : Impressions, CTR, Conversions, Cost per acquisition
Email Campaigns : Delivery Rate, Open Rate, CTR, Conversion rate