Case History -
Details of the patients life circumstances in general and specific particulars of his presenting problem. Usually taken before treatment commences it can provide important pointers to the cause and cure of the problem. Never underestimate the value of a detailed case history.
Catalepsy - 
A condition observed in some forms of mental illness and also a phenomenon obtainable by hypnosis, where a patients limb or limbs becomes rigid and can be placed in any position, where it will remain. 
Catharsis - 
This word literally means 'purging' and describes the process of releasing repressed or pent-up emotional energy. This is usually affected by 'reliving', re-experiencing, acting out or talking out the memories of causal events.
Censor - 
According to psychoanalysis this is a psychological 'mechanism' which acts as a kind of filter or barrier to prevent repressed material or impulses from coming into consciousness.
Cephalagra - 
The correct term for headache. So remember, the next time you take a day off work sick, you didn't just have a headache....you had cephalagra!
Charming - 
Pre-Mesmer hypnosis, also animal hypnosis (re: snake charming etc)
Chevreul's Pendulum -
A simple method of determining or increasing a patient's suggestibility. A small pendulum is held over paper on which a cross (two intersecting lines) are drawn. Then the patient begins to swing the pendulum along one of the lines, while the hypnotist suggests that it will begin to gradually move from it's path until it is swinging along the path of the other line.
Claustrophobia - 
Morbid fear of enclosed spaces
Closure - 
The completion of a psychological process. Developed in Gestalt psychology.
Complex -
A psychological matrix of related emotional material. Term originating with Jung.
Compulsion -
Where a patient feels an irresistible urge to carry out an act, whether a thought or a pattern of behaviour, even against his will. (re:compulsive behaviour). 
Concentration -
The fixing of attention in one place or on one thing.
Conditioned Reflex - 
Is where an action is carried out in response to a trigger because the action and the trigger (stimuli) have become associated (conditioned). Term originating with Ivan P. Pavlov.
Classical Conditioning - 
The process of associating a stimulus with a response.
Conscience - 
A persons moral censor which guides that persons conduct. Cannot be overridden with hypnosis.
Contrasuggestibility -
A curious tendency in some individuals (rare) to respond to a suggestion by acting out the opposite of it's intention.
Coprolalia - 
Compulsion to speak obscenities
Coprophobia - 
Irrational excessive dread of Faeces.
Coue, Emile - 
Pioneer of auto-suggestion (1857 - 1926) and originator of the famous formula, "Day by day and in every way I am getting better and better".
Counter Suggestion - 
A suggestion given to neutralize a previous suggestion or belief.
Critical Faculty -
The ability to make a decision regarding the validity of a particular thing depends upon the exercise of the critical faculty. It is associated with the conscious mind and left hemisphere of the brain. Absence, of the critical faculty means that all 'proposals' are accepted as valid and as such is the temporary goal of hypnosis.
Dreams are a good example of the state of the mind with the critical faculty in abeyance, as the most improbable things can take place in them but they seem perfectly realistic at the time.