Freud's theory of the drive was revised extensively throughout his career.
The drive, or instinct as it is usually translated in English, is a concept that exists on the border between the somatic (bodily) and the mentral.
It consists of a quantity of energy and its psychical representative.
The Freduain drive is a "constant force of a biological nature, emanating from organizc sources, that always has as its aim its own satisfaction through the elimination of the state of tension which operates at the source of the drive itself.
According to Freud, there are four chracterisitcs of the drive: its 'pressure', its 'aim', its 'object' and its 'source.'
By pressure Freud means the drive's motor factor, that is to say, the amount of force or meaasure of the demand for work which it represents.