Compulsion is the ability to force one's will upon someone else. All Moroi possess a relatively standard level of compulsion, although there is an exception with spirit users, who are able to use compulsion almost as effectively as Strigoi. An example of this exception is Lissa Dragomir, who uses her heightened compulsion in many situations throughout the series.
It is implied throughout the series that spirit users are usually able to sense when they are being compelled, and therefore, able to resist it more effectively. Evidence of this is in Last Sacrifice, when Sonya Karp senses Robert Doru is attempting to compel her for information on Eric Dragomir's illegitimate child; she becomes angry, and a fight between spirit users ensues.