Earned media is a term generally used for referring to the POEM representation which differentiates paid, owned and earned media.
Online earned media are free media exposures gained mainly via digital word of mouth.
Online earned media are:
social recommendations (likes, tweets / retweets, pins, etc.)
shared and embedded video (viral video)
Offline earned media are mainly "traditional" word of mouth and free press coverage.
Image source Leaderlab