What is a thriller?

Many people ask what is a thirller? To me a thriller is a film that builds up tension. A thriller can be like a horror as they both build up tension . they might also explore the unknown or the unexpected. A good example is scream. This movie builds up tension by how you are an on looker watching the muders gruadully happen. Another way it builds up thriller is the way you do not see the killers identity.
Horror explores mainly the unknown and paranormal. This is my definition of a thriller