What is Accomplice?

Legal Definition
Under the English common law, an accomplice is a person who actively participates in the commission of a crime, even if they take no part in the actual criminal offense. For example, in a bank robbery, the person who points the gun at the teller and asks for the money is guilty of armed robbery. Anyone else directly involved in the commission of the crime, such as the lookout or the getaway car driver, is an accomplice, even if in the absence of an underlying offense keeping a lookout or driving a car would not be an offense.

An accomplice differs from an accessory in that an accomplice is present at the actual crime, and could be prosecuted even if the main criminal (the principal) is not charged or convicted. An accessory is generally not present at the actual crime, and may be subject to lesser penalties than an accomplice or principal.

At law, an accomplice has lesser guilt than the person he or she is assisting, is subject to lesser prosecution for the same crime, and faces the smaller criminal penalties. As such, the three accomplices to the bank robbery above can also to a degree be found guilty of armed robbery even if only one stole money.

The fairness of the doctrine that the accomplice is still guilty has been subject to much discussion, particularly in cases of capital crimes. Accomplices have been prosecuted for felony murder even if the actual person who committed the murder died at the crime scene or otherwise did not face capital punishment.

In jurisdictions based on the common law, the concept of an accomplice has often been heavily modified by statute, or replaced by new concepts entirely.
-- Wikipedia
Legal Definition
A person who knowingly, voluntarily, or intentionally gives assistance to another in (or in some cases fails to prevent another from) the commission of a crime. An accomplice is criminally liable to the same extent as the principal. An accomplice, unlike an accessory, is typically present when the crime is committed.
Illustrative caselaw
See, e.g. Waddington v. Sarausad, 129 S.Ct. 823 (2009).
See also
Legal Definition
Crim. law. This term includes in its meaning, all persons who have been concerned in the commission of a crime, all particepes crimitis, whether they are considered in strict legal propriety, as principals iu the first or second degree, or merely as accessaries before or after the fact. Foster, 341; 1 Russell, 21; 4 Bl. Com. 331; 1 Phil. Ev. 28; Merlin, Repertoire, mot Complice. U. S. Dig. h. t.

2. But in another sense, by the word accomplice is meant, one who not being a principal, is yet in some way concerned in the commission of a crime. It has been questioned, whether one who was an accomplice to a suicide can be punishhed as such. A case occurred in Prussia where a soldier, at the request of his comrade, had cut the latter in pieces; for this he was tried capitally. In the year 1817, a young woman named Leruth received a recompense for aiding a man to kill himself. He put the point of a bistouri on his naked breast, and used the hand of the young woman to plunge it with greater force into his bosom; hearing some noise he ordered her away. The man receiving effectual aid was soon cured of the wound which had been inflicted; and she was tried and convicted of having inflicted the wound, and punished by ten years' imprisonment. Lepage, Science du Driot,c h. 2 art. 3, 5. The case of Saul, the king of Israel, and his armor bearer, (1 Sam. xxxi. 4,) and of David and the Amelekite, (2 Sam. i. 2-16,) will doubtless occur to the reader.
-- Bouviers Law Dictionary
Legal Definition
In criminal law. A paason who knowingly, voluntarily, and with common intent with the principal offender udltes ip the commission of a crime. Clapp v. State, 94 Tenn. 186, 30 S. W. 214; People v. Bolanger, 71 Cal. 17, 11 Pan. 799; State v. Umide, 1U> Mo. 452, 22 S. W. 378; Carroll v. State, M> Ark. 539; State v. Light, 17 Or. 358, 21 Fine. 132.

One who is pitied or united with another; one of several concerned in a felony; an associate in a erime; one who co-operates, aids, or assists,in committing it State v. Ban, 90 Iowa, jp, 58 N. W. 898. This term includes all jhe "participes criminis, whether considered in strict legal propriety as principals or as accessaries. 1 Buss. Crimes, 26.

It is generally applied to those who are admitted to give evidence against their fellow criminals. 4 Bl. Comm. 331; Hawk. P. C. bk. 2, c. 37, ยง 7; Cross v. People, 47 111. 158, 95 Am. Den. 474.

One who is to some way concerned in the commission of a crime, though not as a principal; and this includes all persons who have been concerned in its commission, whether they are considered, in strict legal propriety, as principals in the first or second degree, or merely as accessaries before or after the fact. In re Rowe, 77 Fed. 161, 23 C. C. A. 103; People v. Bolanger, 71 Cal. 17 11 Pac. 790; Polk v. State, 36 Ark. 117; Armstrong v. State, 33 Tex. Cr. IS. 417, 26 S. W. 829.
-- Black's Law Dictionary
Legal Definition
One so connected with the crime that at common law he might himself have been convicted either as principal or as an accessory before the fact. See Ann. Cas. 1913A, 771; also 20 Am. St. Rep. 163 and 39 L. R. A. (N. S.) 704, and note.
-- Ballentine's Law Dictionary