As defined in the psychology, sociology and social work fields, a vulnerable adult is any person who lacks the absolute most basic (as distinct from mid-level or typical level) human life skills by reason of not having learned them through the formative years of childhood, adolescence and young adulthood. A vulnerable adult is unable, rather than unwilling, to properly learn or properly maintain these skills, and is usually completely without, and unable to obtain, any family, friends, acquaintances or other assistive persons in their lives to offer education or assistance in these areas (see: abandonment). In order to be classed as vulnerable, the adult's circumstances must be unable to be altered or improved by the adult's own individual actions without direct assistance from a more typical adult. The vulnerable adult must also be shown to be, on some significant level, a risk to him or her self if assistance is not provided.
England and Wales
In the law of England and Wales a wide definition is applied to meet the standard of vulnerable adult. Section 59 of the Safeguarding Vulnerable Groups Act 2006 states that:
A person is a vulnerable adult if, having attained the age of 18, s/he —
- is in residential accommodation,
- is in sheltered housing,
- receives domiciliary care,
- receives any form of health care,
- is detained in lawful custody,
- by virtue of an order of a court, is under supervision per Criminal Justice Act 2003 sections regarding community sentences;
- receives a welfare service of a prescribed description,
- receives any service or participates in any activity provided specifically for persons who has particular needs because of his age, has any form of disability or has a prescribed physical or mental problem. (Dyslexia, dyscalculia and dyspraxia are excluded disabilities) ,
- has payments made to him/her or to an accepted representative in pursuance of arrangements under Health and Social Care Act 2012, and/or
- requires assistance in the conduct of own affairs. 
In most parts of the world, the last section is the usual headlining definition for a vulnerable adult, i.e. the adult is unable to function cognitively or to adequately undertake basic day-to-day functions without the help or oversight of someone not impaired in these ways.
Diagnosis and treatment
A vulnerable adult, if seen by a doctor for a long enough period of time, is usually given an official clinical diagnosis of being a vulnerable adult. The purpose of such a diagnosis is to ensure that a relevant social work department, housing authority, etc. (if these exist in the relevant country) is/are able to enter the life of the vulnerable adult for assistive purposes.
If the vulnerable adult has been abused, which is typically the case for long-term vulnerability, trauma counselling and an assisted living facility may also be offered to the vulnerable adult by the relevant authority or authorities. Despite all this, some vulnerable adults are indeed long-term homeless.
A main cause of being a vulnerable adult is usually a clinical-level cognitive impairment such as Down syndrome but can also be caused and/or exacerbated by other cognition issues and/or the long-term effects of abuse and severe neglect from an early age within a family structure. Some combination of abuse, severe neglect, and cognitive impairments, rather than any one of these things alone, is usually required for a vulnerable adult to become sufficiently vulnerable to be classed as a vulnerable adult.
A vulnerable adult's activities of daily living are usually impaired. Mild forms of such impairment include not knowing or being able to learn the skills necessary to communicate with a local government authority when help is needed (requiring an appropriate adult to step in to advocate on the vulnerable adult's behalf); not knowing how and not being able to learn to read or write complex documents when required such as letters from a court or a debt collector and thus avoiding them rather than seeking help to resolve them; not knowing how to navigate basic money management or personal finance; and so on. Severe forms of such impairment are myriad and are usually too complex to precisely define, but can include very severe learning disabilities together with cognitive impairments limiting the ability of the vulnerable adult to either give or receive human communication with another adult who does not have such impairments.
Vulnerable adults often are assigned to independent or semi-independent living situations inside assisted living facilities or even "community-supported living" council estates, but depending on the resources of the country in question, and also the interpretation by a government authority of the precise degrees of vulnerability, the vulnerable adult is sometimes restricted to a 'residential home' (a quasi-hospital living environment) or is assigned long-term hospitalization.
Problems sometimes arise as to the exact legal status of a vulnerable adult when there is a clear case of the vulnerable adult existing in a "grey area" between mild and severe overall impairment. Sometimes, even when in some ways severely impaired, a vulnerable adult may still be competent enough to not be regarded by the relevant authorities as sufficiently childlike for long-term care. Guardianship is also a problem when the vulnerable adult is completely without friends, family or acquaintances and legally-important matters (such as paperwork) must be properly filled out, but the vulnerable adult is unable to understand what the paperwork contains or why it is important. In the United Kingdom, these situations are sometimes worsened by the Data Protection Act which prevents even some officials and/or appropriate adults from accessing the vulnerable adult's complete personal information for assistive purposes. Some countries solve these issues by getting the vulnerable adult to sign power of attorney over to a relevant authority's representative, a solicitor providing the vulnerable adult free legal help, or similar.