Trial marketing is the process of offering a service for free for a limited amount of time or content and hoping to convert trial users into paying users after the deadline or threshold.
Trial marketing is used for softwares and online services through trial periods and free units. Paywall model for news sites is a kind of trial marketing.
Trial period is cost effective for digital goods where distribution of the free service or content is almost done at no cost.
Fremium model is also a kind of trial marketing wherein the free service is a little downgraded or have less features versus the paid one.
Trial marketing is not specific to digital marketing as it has been used in off-line marketing for a long time via samples. But in traditional marketing samples are costly.
An example of free trial for high ticket items :