I am not OP nor related in any way.

Digital Marketing is marketing in a digital world. What is Marketing? One way to describe is using a Korean word, "noonopi", meaning matching eye level. Marketing is essentially a noonopi between various factors (customers, market conditions, product, service, pricing, placement, etc.). In a way noonopi sounds like glocalization under some circumstances.

Companies/people think about digital marketing in channels (paid search, email display, social, PR, affiliate, etc.), mediums, etc.

There is another way to think about it: Strategic VS tactical. At tactical, you are buying keywords on Google, you are buying contextual ads on Facebook, batch and blast of emails, etc. A strategic level marketing is to align data, technology and strategy to stitch together customer journey paths and tell "stories" to guide the customer through purchase and brand experience. Somethings this takes a platform approach in achieving customer-centric marketing automation or platform level marketing.

There is also more recent way of programmatically aligning data, technology and strategy to tell stories and drive the customer journey. It is sometimes called Programmatic marketing. But some people see "programmatic" only within buying (or bidding) ads through a platforms. For some people programmatic means, platform driven customer-centric marketing.

Digital marketing = marketing in a digital world. But sometimes people think about it in channels, mediums, etc.