(Literature) the flaw in character which leads to the downfall of the protagonist in a tragedy
ETYMOLOGY: 19th Century: from Greek (1)

"hamartia : (GK 'error') Primarily, an error of judgement which may arise from ignorance or some moral shortcoming. Discussing tragedy and the tragic hero in Poetics, Aristotle points out that the tragic hero ought to be a man whose misfortune comes to him, not through vice or depravity, but by some error. For example: Oedipus kills his father from impulse, and marries his mother out of ignorance. Antigone resists the law of the state from stubbornness and defiance. Phèdre is consumed by her passion for Hippolyte." (2)

Interestingly the Romans used the same word as an archery term - To miss the target; Literally implying slight failure (deviation) from the central target point (aim).  The target was often so far away that the archer could not see how he had performed so the cunning Romans put a ditch in front of the target and a slave in the ditch.  The slave would then look at the arrow and call out "H'amartia" if the arrow had missed the main target.  Used in this way it implies a shortcoming of a possibly very slight degree which has lead to failure. This leads us to - "to fail or fall short of that which is expected, to sin".  The way we translate the word today is into the word "sin" we would say (were we the slave) "It is a sinner".

The Word sin was first used in a lifestyle context by a man called Paul (aka St Paul) who's letters make up a bulky part of the new testament section of the Bible. He said "...for all have sinned and fallen short of the glory of God", he knew full well what he meant when he said "all have sinned..." and so did his original readers but nowadays this knowledge is a little harder to come by.  The word sin in that case was of course hamartia.

An action that is wrong is hamartia.  The interesting subtlety of this word gives no room for degrees there is no such thing as being less wrong or lower grades of hamartia one hits the target one aimed for or one fails. We could say this is a simple Boolean event.   If I clip the target or miss by three miles I still failed.

"According to Aristotle, the tragic hero must fall through his or her own error, or hamartia. This term is also interpreted as "tragic flaw" and usually applied to overweening pride, or hubris, which causes fatal error." (3) 

"The classic example of Aristotelian principles is Sophocles' Oedipus the King (ca. 428 B.C.); Shakespeare's Othello (1603-04) follows a similar pattern of pride, error, and self-destruction (though Oedipus merely mutilates himself on discovering his crimes, whereas Othello commits suicide)." (3)

"Recent scholarship has suggested that the interpretation of hamartia as a fatal flaw is itself flawed, and that the word more properly means any disproportion in the character's makeup that leads to downfall; thus an excess of a valuable or virtuous quality can in some circumstances be seen as hamartia." (3)

I find it particularly ironic that the classical interpretation of hamartia should itself be subject to hamartia but it is the implication to Christian dogma that I find most interesting: if "an excess of a valuable or virtuous  quality can... be seen as hamartia." then this brings much of (if not the Church itself then) the generally accepted man-on-the-street concept of sin into question.  One could see the logical line of argument to say that: sin is anything within a person that causes deviation from perfect well roundedness and balance.  The actions leading from that imbalance would merely be a practical extension of that sin.  The implications of this thought are staggering - seemingly saintly people become common "sinners" just like you and me.  To ask "What must I do to be sin free?" one might as well ask "How can I be utterly perfect?".  

Interestingly Hamartophobia is fear of sinning but sometimes I imagine it to be fear of sin...

tdent rightly points out: "Jesus' remark about 'whited sepulchres' shows that he understood this already: sin is not only an external action it is an internal failing."
(For another quote from Jesus on this subject see also: Sin)

(3) The UVic Writer's Guide.

Other Sources
For those that call for more points of reference: (I know the www proves nothing but):
also highly recommended to me:  Elaine Pagel's book "The Gnostic Gospels"