Developmental delay is defined as failure to meet expected developmental milestones in one or more of the following areas: physical, social, emotional, intellectual, speech and language and/or adaptive development (sometimes called self-help skills, which include dressing, toileting, feeding, etc).

It is diagnosed when a child performs approximately 25 to 30% below age norms in one or more of these areas (with adjustment for prematurity in affected children). Progress occurs at a slower than expected rate following the anticipated sequence. Various medical and environmental causes exist.

Some examples of physical or mental disorders likely to result in delayed developmental are: chromosomal abnormalities; genetic or congenital disorders; severe sensory impairments, including hearing and vision; inborn errors of metabolism; disorders reflecting disturbance of the development of the nervous system; congenital infections; and disorders secondary to exposure to toxic substances, including fetal alcohol syndrome.

The verification of delay is obtained through an evaluation process which includes at least three of the following: informed clinical opinion to include observational assessment, standardized development test(s), developmental inventory, behavioral checklist, adaptive behavior measure, and parent interview. Developmental delay can occur temporarily or it can be long-term and never fully resolve.