(Latin: in, not; peccare, to sin)
The impossibility of sinning.
Christ's human will was impeccable because of its personal union with a Divine Person.
A Divine Person cannot from its very nature possess the faculty of committing sin.
Concupiscence and actual sin flow from original sin which we contract by reason of our carnal descent from Adam, but Christ was conceived miraculously through the operation of the Holy Ghost.
Christ Himself said: "Which of you shall convince me of sin?" (John 8).
This word is often confused with infallibility.
Impeccability has to do with the will and with sin; infallibility, with the intellect and with opinion or doctrine.
The pope is infallible, but not impeccable.
New Catholic Dictionary