Definition or reference in primary legislation
Mental Capacity Act 2005, S.2
(1) For the purposes of this Act, a person lacks capacity in relation to a matter if at the material time he is unable to make a decision for himself in relation to the matter because of an impairment of, or a disturbance in the functioning of, the mind or brain.
(2) It does not matter whether the impairment or disturbance is permanent or temporary.
(3) A lack of capacity cannot be established merely by reference to--
(a) a person's age or appearance, or
(b) a condition of his, or an aspect of his behaviour, which might lead others to make unjustified assumptions about his capacity.
ASC Collections Data Dictionary (Beta v1.3)
Professionals must work in line with the Mental Capacity Act (MCA) and where an MCA assessment concludes an individual lacks capacity to make decisions about the safeguarding incident, decisions made on their behalf must always be in their best interest.
Source: Social worker with an integrated hospital team background
A person's mental capacity may fluctuate over time and may vary depending on the decision to be made. This isn't always understood or recognised by health professionals, although the definitions here are reasonably consistent about it. This can lead to people being inappropriately prevented from making decisions which they do have the capacity to make.