I'd have thought by now that there would have been many others offering their opinions. Since nobdy else wants to answer, I'll guess I'll be first.
Sin is "missing the mark" the "mark" being perfection as in Matthew 5:48. "Be perfect as your Father in Heaven is perfect". Anything less than this perfection is sin, inside and out.
"Hamartia" is the word ususally translated as "sin". It refers to not only the actions but the inward element that can lead to outwardly sinful acts.