A communication disorder is any disorder that affects an individual's ability to comprehend, detect, or apply language and speech to engage in discourse effectively with others. The delays and disorders can range from simple sound substitution to the inability to understand or use one's native language.

Disorders and tendencies included and excluded under the category of communication disorders may vary by source. For example, the definitions offered by the American Speech–Language–Hearing Association differ from that of the Diagnostic Statistical Manual 4th edition (DSM-IV). Gleanson (2001) defines a communication disorder as a speech and language disorder which refers to problems in communication and in related areas such as oral motor function. The delays and disorders can range from simple sound substitution to the inability to understand or use their native language. In general, communications disorders commonly refer to problems in speech (comprehension and/or expression) that significantly interfere with an individual’s achievement and/or quality of life. Knowing the operational definition of the agency performing an assessment or giving a diagnosis may help. Persons who speak more than one language or are considered to have an accent in their location of residence do not have speech disorders if they are speaking in a manner consistent with their home environment or a blending of their home and foreign environment. .

Classifying speech into normal and disordered is more problematic than it first seems. By a strict classification, only 5% to 10% of the population has a completely normal manner of speaking (with respect to all parameters) and healthy voice; all others suffer from one disorder or another. There are three different levels of classification when determining the magnitude and type of a speech disorders and the proper treatment or therapy: 1. Sounds the patient can produce 1. Phonemic – can be produced easily; used meaningfully and constructively 2. Phonetic – produced only upon request; not used consistently, meaningfully, or constructively; not used in connected speech 2. Stimulate sounds 1. Easily stimulated 2. Stimulate after demonstration and probing (i.e. with a tongue depressor) 3. Cannot produce the sound 1. Cannot be produced voluntarily 2. No production ever observed