In criminal law, guilt is the state of being responsible for the commission of an offense. Legal guilt is entirely externally defined by the state, or more generally a "court of law". Being "guilty" of a criminal offense means that one has committed a violation of criminal law, or performed all the elements of the offense set out by a criminal statute. The determination that one has committed that violation is made by an external body (a "court of law") and is, therefore, as definitive as the record-keeping of the body. So the most basic definition is fundamentally circular: a person is guilty of violating a law, if a court says so.
Philosophically, guilt in criminal law is a reflection of a functioning society and its ability to condemn individuals' actions. It rests fundamentally on a presumption of free will, in which individuals choose actions and are, therefore, subjected to external judgement of the rightness or wrongness of those actions.
An adjudication of guilt is more than a factual determination that the defendant pulled a trigger, took a bicycle, or sold heroin. It is a moral judgment that the individual is blameworthy. Our collective conscience does not allow punishment where it cannot impose blame. Our concept of blameworthiness rests on assumptions that are older than the Republic: man is naturally endowed with these two great faculties, understanding and liberty of will. Historically, our substantive criminal law is based on a theory of punishing the viscious [sic] will. It postulates a free agent confronted with a choice between doing right and wrong, and choosing freely to do wrong.
See also Cotton, Michael, A FOOLISH CONSISTENCY: KEEPING DETERMINISM OUT OF THE CRIMINAL LAW, 15 B.U. Pub. Int. L.J. 1 ("A substantial body of scholarship has concerned itself with the importance of free will to the theory of the criminal law. Even given the importance of the subject, the quantity of attention is surprising because of the lack of fundamental disagreement among scholars, who overwhelmingly endorse the criminal law's assumption of free will.")
"Guilt" is the obligation of a person who has violated a moral standard to bear the sanctions imposed by that moral standard. In legal terms, guilt means having been found to have violated a criminal law, though law also raises 'the issue of defences, pleas, the mitigation of offences, and the defeasibility of claims'.
Les Parrott draws a three-fold distinction between "objective or legal guilt, which occurs when society's laws have been broken... social guilt...[over] an unwritten law of social expectation", and finally the way "personal guilt occurs when someone compromises one's own standards".