The mark of the beast is described in Revelation 13:16-17. It is:

Placed on the right hand or forehead

Required for buying and selling

Associated with the name of the beast or the number of its name

The Number 666

Revelation 13:18 states, "This calls for wisdom. Let the person who has insight calculate the number of the beast, for it is the number of a man. That number is 666."1