The porn video tag "mtf-transgender" refers to content that features a transgender individual assigned male at birth who is transitioning or has transitioned to living as a woman. In this context, "mtf" stands for "male to female." The tag signifies that the viewer can expect to see scenes involving individuals who identify as trans women or gender-nonconforming individuals in various sexual situations, and it caters to those who have a specific interest in transgender pornography.