undetectable equals untransmittable

The USA's national health protection agency, known as the Centers for Disease Control and Prevention on September 27, 2017, officially released the historic report stating "When ART results in viral suppression, defined as less than 200 copies/ml or undetectable levels, it prevents sexual HIV transmission." -Eugene McCray, MD CDC

This news is the motivator for the now commonly used term, Undetectable Equals Untransmittable.

So undetectable equals untransmittable means that if your HIV viral count is undetectable, then essentially you're not a risk to pass the infection to another.