What is a mental disorder or developmental disability rendering a defendant incompetent to stand trial?

"A defendant is incompetent to stand trial when he suffers a mental disorder or developmental disability rendering him ' unable to understand the nature of the criminal proceedings or to assist counsel in the conduct of a defense in a rational manner.' [Citations.] As a matter of due process, the trial court is required to conduct a section 1368 hearing to determine a defendant's competency whenever substantial evidence of incompetence has been introduced. [Citations.] Substantial evidence is evidence that raises a reasonable doubt about the defendant's competence to stand trial. [Citations.]" (People v. Frye (1998) 18 Cal.4th 894, 951-952, 959 P.2d 183.)