Final answer:
The illegal defense where an offender cannot comprehend the charges or assist in their defense is termed 'incompetence to stand trial'. This is upheld by the U.S. legal system, underpinning the right to counsel for the accused as established in landmark cases like Gideon v. Wainwright.
Step-by-step explanation:
The illegal defense that claims the offender is unable to understand the charges or assist legal counsel in preparing a defense is known as a claim of incompetence to stand trial. The legal standard for determining incompetence is whether the defendant has a rational and factual understanding of the proceedings against them and whether they can rationally consult with their lawyer.
This concept is deeply embedded in the United States legal system, as highlighted by landmark cases such as Gideon v. Wainwright, which secures the right of the accused to legal counsel, particularly when they are unable to afford one, and Strickland v. Washington, which set the standard for determining ineffective assistance of counsel.