What is the meaning of the word "fasiq" (transgressor), and is it permissible for us to describe whomever we wish as such? What are the criteria for judging someone to be a fasiq?

Linguistically, fisq (transgression) means to exit, like a date exiting its shell. Islamically, it is committing major sins or persistently committing minor sins without a valid interpretation. The fundamental rule regarding backbiting (gheeba) is its prohibition, based on the Almighty's saying: "And do not backbite each other." An exception to this is the permissibility of backbiting a transgressor (fasiq) concerning the evil he openly declares, or for the purpose of warning people against him. However, it is not permissible to backbite him concerning anything other than his transgression, or to attribute to him something that has not been proven.