What is the definition or description of: apraxia?

Unable to perform: Apraxia is a disorder of the brain and nervous system in which a person is unable to perform tasks or movements when asked, even though: •the request or command is understood •they are willing to perform the task •the muscles needed to perform the task work properly •the task may have already been learned see: http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0004944/.

Inability to perform: Apraxia refers to several concepts. One is where motor or cognitive tasks cannot be done even when comprehension and physical capacity to perform are present, often referring to acquired difficulties, sometimes inborn. The term may be used to describe problems seen in daily living, or the failure to complete a task during testing. It's not generally applied to ignorance or extraordinary challenges.