It’s is advertising delivered through digital channels. Channels such as social media, mobile applications, email, web applications, search engines, websites, or any new digital channel.

Digital marketing is any form of marketing products or services that involves electronic devices.
That’s the reason it has been around for decades (because electronics have) and why it doesn’t necessarily have anything to do with content marketing, Google ads, social media or retargeting.
Digital marketing can be done both online and offline.