By defect I suppose you are refering to a weakness in her character that results in the problems that she faces, or a negative quality that she possesses. It is clear that Abigail Williams is a character with a number of "defects" in this sense, but certainly her chief defect is her propensity to lie. Let us remember how she is introduced when she first enters the play at the bedside of Betty Parris in Act One. She is described as being:
...a strikingly beautiful girl, and orphan, with an endless capacity for dissembling.
This is clearly her biggest defect. She is above all else an actress, and she shows her dramatic talent in a number of separate instances, managing to convince nearly everyone that she is sincerely repenting and revealing the names of those in league with the devil and that she is suffering demonic attacks as a result of various indivduals and their relationship with the devil. Abigail's biggest defect is something that she tries to use to gain what she wants, which is John Proctor.