What is a "Holy Kiss?"

Honestly, I have to say that I have asked myself that question times without number and I still do not know the answer to it! I know what a kiss is most definitely but what is a "Holy Kiss?" Please refer to Romans 16:16 — "Greet one another with a holy kiss"

Just when does a kiss start to be holy 'cause I know when it ceases to be all right! ;)
Someone should please ask Pastor for me...no, seriously!