Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behavior

Adaptive behavior

Adaptive behavior is a type of behavior that is used to adjust to another type of behavior or situation. This is often characterized by a kind of behavior that allows an individual to change an unconstructive or disruptive behavior to something more constructive. These behaviors are most often...

s. It has historically been defined as an Intelligence Quotient

Intelligence quotient

An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...

 score under 70.
Once focused almost entirely on cognition

Cognition

In science, cognition refers to mental processes. These processes include attention, remembering, producing and understanding language, solving problems, and making decisions. Cognition is studied in various disciplines such as psychology, philosophy, linguistics, and computer science...

, the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environment.