In it’s simplest form a Viral Loop is term used for growing your product via referrals.
For example, person uses product -> tells friend -> the friend uses the product -> tells another friend who uses the product -> cycle is repeated.
Defining the viral loop served the purpose of showing how a product might grow sustainably and allow teams to visualise how they might begin to implement a viral element into their product.
Viral loops can happen organically but most of the time companies choose to instigate viral growth by altering the product in some way.
A couple of good examples would be…
There are other viral elements which determine the effectiveness of your Viral-Loop such as the Viral Coefficient & Cycle time.
Viral Coefficient: A measure of Virality
Cycle Time: The average time it takes to complete a viral-loop.