Someone you have never met IRL (in real life) but have corresponded with online and therefore have a "digital" relationship with.
Often abbreviated simply as "DC".
I've not been able to determine where or when this term was first used, but I was made aware of it from reading a blog entry at www.My-CreativeTeam.com
Might have originally meant to describe those you've ONLY corresponded with via online text through SNS's (social networking sites), internet forums (discussion or message boards), E-mail, IM's (instant messaging), etc... but seems to have also grown to include those you correspond with via voice through VOIP lines (as in Skype or broadband phones), cell phones, or landline phones.
I am a real estate agent and have lots of digital colleagues over on ActiveRain.com where we all interact with each other for venting, brainstorming, advice, and business referrals.
We have the confidence to refer our clients to each other because we've built trust through ongoing, online relationships with each other by blogging, comments, and private messages back and forth even though we're all in different states and provinces and might never meet each other face to face.