"Intellectual disability" is the term used in Australia to describe individuals who have impaired cognitive functioning evidenced in childhood. The same condition is described as "learning disability" in the United Kingdom and "mental retardation" in the United States.

The legal definition of intellectual disability in Victoria is found as item (b) of the definition of "disabilities" in the Definitions section (s.3) of the Disability Act 2006 (Vic) ("Disability Act") (you can also find the definition of "disabilities" under "Disability Act 2006" in Chapter 16.1 Disability Overview):

Note: Before 1 July 2006, the same definition was contained in the Intellectually Disabled Persons' Services Act 1986 (Vic) ("IDPSA") [repealed].

The Disability Act (s.6(3)) sets out the test for determining whether or not a person has an intellectual disability. Sections 4 to 9 set out other important provisions in relation to people who have an intellectual disability.

Current internationally accepted diagnostic criteria for the condition of intellectual disability are more explicit. For example, the Fourth Edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), which is the internationally used manual in relation to diagnosis of mental impairments, states the diagnostic criteria are:

These DSM-IV diagnostic criteria may be useful in arguing before a court that a person has an intellectual disability, but eligibility for intellectual disability services, which are provided or funded by the Victorian Department of Human Services (DHS), is determined by the criteria set out in the Disability Act.  See: "Eligibility for services", below.

Generalisations about intellectual disability, and therefore about the capacity of a person with an intellectual disability, frequently underestimate a person's abilities. Intellectual disability may limit a person's functional skills, ability to learn and understanding of concepts, but there are many degrees of intellectual disability. Many people who have an intellectual disability are able to function very well, others may need some assistance with some aspects of their life, while yet others may need significant assistance.

It is vital that each person's case be assessed individually and that the views of the person who has an intellectual disability always be sought, taken into account and, wherever possible, acted upon. It should be noted that the aim of the Victorian Government's State Disability Plan is for all people with a disability to be fully included in the life of the community and to have the same opportunities as people who do not have a disability. About 1% of the population of Victoria have an intellectual disability, approximately 49,000 people. In 2001, DHS provided services to 9,614 people who have an intellectual disability.

In the past, there has been general confusion between intellectual disability and mental illness. The Disability Act (and the IDPSA prior to 1 July 2007) and the Mental Health Act 1986 (Vic) ("MHA") make a clear distinction between the two (s.8 MHA).

Some people may have both an intellectual disability and a mental illness. This is often termed "dual disability" or "dual diagnosis".

Until 1 July 2007, when the Disability Act came into effect, no specific legislation dealt with the provision of services to people with other forms of cognitive impairment, such as acquired brain injury (cognitive impairment caused by accidents, organic brain injury or drug use) or some autism spectrum disorders (see below). People who had these, or other, disabilities may have accessed services funded under the Disability Services Act 1991 (Vic) [repealed] ("DSA (Vic)") and the Disability Services Act 1986 (Cth) ("DSA (Cth)"). The Disability Act extended this coverage to include people with some other forms of cognitive impairment, such as acquired brain injury.