A learning disability exists when there is a significant discrepancy between one's ability and achievement. Usually this discrepancy equals a 1.5 standard deviation, typically 22 points between the IQ and an academic area such as math, reading, or written language. Someone with a learning disability does not necessarily have low or high intelligence, it just means this individual is working far below their ability due to a processing disorder, such as auditory processing or visual processing. Learning disabilities are usually identified by school psychologists through testing of intelligence, academics and processes of learning.

Official definitions
The National Joint Council on Learning Disabilities (United States) defines it this way:

"Learning Disability" is a generic term that refers to a heterogeneous group of disorders manifested by significant difficulties in the acquisition and use of listening, speaking, reading, writing, reasoning or mathematical abilities. These disorders are intrinsic to the individual and presumed to be due to Central Nervous System Dysfunction. Even though a learning disability may occur concomitantly with other handicapping conditions (e.g. sensory impairment, mental retardation, social and emotional disturbance) or environmental influences (e.g. cultural differences, insufficient/inappropriate instruction, psychogenic factors) it is not the direct result of those conditions or influences.

The Individuals with Disabilities Education Act (United States) defines a learning disability this way:

disorder in one or more of the basic psychological processes involved in understanding or in using spoken or written language, which may manifest itself in an imperfect ability to listen, think, speak, read, write, spell, or to do mathematical calculations. . . .Learning disabilities include such conditions as perceptual disabilities, brain injury, minimal brain dysfunction, dyslexia, and developmental aphasia.

Contrast with other conditions
People with an IQ lower than 70 are usually characterised as having mental retardation (MR), mental deficiency, or cognitive impairment and are not included under most definitions of learning disabilities, because their learning difficulties are related directly to their low intelligence. In contrast, learning disabled individuals have the potential to learn as much as other people of average intelligence, but something is preventing them from reaching that potential.

In the UK, learning disability is often confusingly used as a synonym of mental retardation, due to the social stigma attached to the latter term. However, this is not internationally recognised, and the correct term for people with an IQ below 70 remains mental retardation.

Attention-deficit hyperactivity disorder (ADHD) is often studied in connection with learning disabilities, but it is not actually included in the standard definitions of learning disabilities. It is true that a person with ADHD struggles with learning, but he can often learn adequately once he or she is successfully treated for the ADHD. A person can have ADHD but not be learning disabled or be learning disabled without having ADHD.

Types of learning disabilities
Learning disabilities involve many areas of perception, which include:

1. Visual or Auditory Discrimination - perceiving differences in either sights or sounds
2. Visual or Auditory Closure - filling in missing parts of sights or sounds
3. Visual or Auditory Figure-ground Discrimination - focusing on an object and disregarding its background
4. Visual or Auditory Memory, either short-term or long-term
5. Visual or Auditory Sequencing - putting what is seen or heard in the right order
6. Auditory Association and Comprehension - relating what is heard to other things, including definitions of words and meanings of sentences
7. Spatial Perception - laterality (above vs. below, between, inside vs. outside) and one's position in space
8. Temporal Perception - processing time intervals in the range of milliseconds, critical to the development of speech processing

Various terms are used to describe particular learning disabilities. A person can have one of them or more than one of them.

Some of them are as follows:

1. Speech and language disorders

a. difficulty producing speech sounds (articulation disorder)
b. difficulty putting ideas into spoken form (expressive disorder)
c. difficulty perceiving or understanding what other people say (receptive disorder)

2. Dyslexia--the general term for a disability in the area of reading.

a. difficulty in phonetic mapping, where sufferers have difficulty with matching various orthographic representations to specific sounds
b. difficulty with spatial orientation, which is stereotyped in the confusion of the letters <b> and <d>, as well as other pairs. In its most severe form, <b> <d> <g> and <q>, all distinguished primarily by orientation in handwriting, look identical to the dyslexic
c. difficulty with sequential ordering, such that a person can see a combination of letters but not perceive them in the correct order

3. Dysgraphia--the general term for a disability in the area of physical writing. It is usually linked to problems with visual-motor integration or fine motor skills.
4. Dyscalculia--the general term for a disability in the area of math.

An accepted way of referring to these people as "special" is due to their special circumstances. In modern times it is generally regarded as unacceptable to deride or make fun of someone for such a disability.

Possible causes
Various theories have been posited for the cause or causes of learning disabilities. They are thought to involve the brain in some fashion. The most common causes are thought to be:

1. defects or errors in brain structure

2. drug abuse

3. poor nutrition

4. passed on in parents genes

5. lack of parental involvement during early development stages in the infant

6. lack of communication between various parts of the brain

7. incorrect quantities of various neurotransmitters, or problems in the brain's use of these transmitters

common neurotransmitter problems include insufficient dopamine, improper serotonin regulation, and excessive dopamine reuptake where emitting neurons reabsorb too much dopamine after releasing them to communicate with other neurons (also implicated in depression).

Treatment
Learning disabilities are treated with a variety of methods. Some (adjustments, equipment and assistants) are designed to help compensate for the disabilities and others (Special Education) are used to make improvements in the weak areas. Treatments include:

1. Classroom adjustments:

special seating assignments
alternative or modified assignments
modified testing procedures

2. Special equipment:

electronic spellers and dictionaries
word processors
talking calculators
books on tape

3. Classroom assistants:

note-takers
readers
proofreaders

4. Special Education:

prescribed hours in a special class
placement in a special class
enrollment in a special school for learning disabled students