The most relevant definitions of shock on Dictionary.com are:
1.
a sudden or violent disturbance of the mind, emotions, or sensibilities
2.
to strike or jar with intense surprise, horror, disgust, etc.
I believe shock (in the purpose of horror film) to be to scare the audience for a brief moment. Like terror but over a shorterperiod of time. Good horror films are filled with moments that shock the reader and I will also need to shock the audience in my horror opening.
To help study shock in film I watched Un Chien Andalou. This film was released in 1929 and was banned soon after because of the shock it caused on the audience at the time. The shock in this film is caused by the clips which in the time would have been considered very horrific. The main cause of shock is the lady's eye being cut open. The fact it is a human shocks us more as we feel that it could happen to us; we relate to it. I need to cause this same kind of shock in my opening scene.