Unable to make mistake, to do wrong, faultless, sinless,
incapable of erring.
Masumiyet, masum.
Useful information:
The Shi'a school of thought maintains the belief that all the prophets of Allah, from Adam ( pbuh ) to Muhammad ( pbuh&hf ), as well as the twelve successors to the Holy Prophet Muhammad ( pbuh&hf ) and his daughter Lady Fatima al-Zahra' ( pbut ), were infallible throughout their entire lives and never committed any type of sin that would dissatisfy Allah. The clearest way to see this point is to consider that these people were the examples sent for humanity to follow, and so if they committed errors, people would be obliged to follow even their errors, thereby rendering the prophets and messengers unreliable. Infallibility means protection, and, in Islamic terminology, means the spiritual grace of Allah enabling a person to abstain from sins by his own free will. The power of infallibility or sinlessness does not make a person incapable of committing sins; rather, he refrains from sins and mistakes by his own power and will.