Per Lord Brightman in Hart v O'Connor  AC 1000, PC:
If a contract is stigmatised as 'unfair', it may be unfair in one of two ways. It may be unfair by reason of the unfair manner in which it was brought into existence; a contract induced by undue influence is unfair in this sense. It will be convenient to call this 'procedural unfairness'. It may also, in some contexts, be described (accurately or inaccurately) as 'unfair' by reason of the fact that the terms of the contract are more favourable to one party than to the other. In order to distinguish this 'unfairness' from procedural unfairness, it will be convenient to call it 'contractual imbalance'. The two concepts may overlap. Contractual imbalance may be so extreme as to raise a presumption of procedural unfairness, such as undue influence or some other form of victimisation. Equity will relieve a party from a contract which he has been induced to make as a result of victimisation. Equity will not relieve a party from a contract on the ground only that there is contractual imbalance not amounting to unconscionable dealing.