In Wigmore on Evidence, circa 1916, the authors state:
"When a witness has made himself an agent for the prosecution before associating with the wrong doers or before the actual perpetration of the offence, he is not an accomplice; but he may be if he extends no aid to the prosecution until after the offence is committed. A mere detective or decoy is therefore not an accomplice."
In MacDonald v The King, Justice Tashereau of the Supreme Court of Canada went on a legal definition rampage in adopting the words of the trial judge in regards to accomplice:
"The learned trial Judge also explained to the jury what was an accomplice, its legal meaning, and gave various definitions. He said ... an accomplice is one who knowingly and in a common intent with the principal offender, unites in the completion of a crime.
"Or, to determine if a witness is an accomplice, ask this question: Could the witness have been indicted under the wide provisions of the Criminal Code for the offence for whch the person has been convicted or is being tried?
"And other definition: An accomplice is a party to the crime himself, who assists in or is a partner of the crime.
"One more: Every person who knowingly, deliberately co-operates with or assists or even encourages another in the completion of a crime is an accomplice."
In Korell, Justice Henson of the Court of Appeals of Texas at Austin offered this description:
"An accomplice is an individual who participates with a defendant before, during, or after the commission of the crime and acts with the requisite culpable mental state.
"Further, the accomplice witness's participation must involve an affirmative act that promotes the commission of the offense with which the defendant is charged. There must be sufficient evidence to connect the alleged accomplice to the criminal offense as a blameworthy participant."
In The People v Williams, Justice George of the Supreme Court of California wrote:
"[A]n accomplice is one who aids or promotes the perpetrator's crime with knowledge of the perpetrator's unlawful purpose and an intent to assist in the commission of the target crime, and an accomplice may be guilty of the target crime and also of other crimes that are considered the natural and probable consequence of the target crime."
Three months earlier, in the Court of Appeals of Idaho, Justice Perry had used these words in State v Chacon:
"An accomplice is a person involved in the commission of a crime, whether he or she participates directly or indirectly.
"To be an accomplice, it is sufficient to aid and abet, advise, or encourage the commission of the crime.
"The general rule, however, is that one who participates in a crime for the purpose of gathering evidence against another participant is not an accomplice. An informer or agent in the employ of the police who makes a narcotics purchase from a suspected seller is not an accomplice. A person who participates in criminal activity only as an agent of law enforcement lacks the requisite criminal intent to have aided or abetted the commission of the crime.
"A person's status as an accomplice can be decided as a matter of law if it appears without substantial conflict in the testimony that the person participated in, or encouraged, the crime. But where there is uncertainty whether a witness is an accomplice, the issue should be submitted to the jury to be determined as a question of fact."
Similarly, as regards to an individual who was merely present.
In R. v Morris, Justice Ritchie of Canada's Supreme Court wrote (at ¶12):
"[A] person does not become an accomplice merely by witnessing an act and taking no steps to prevent."
In the 14th edition of Roscoe's Criminal Evidence, the author notes:
"An accomplice is a person who has either been convicted of or confessed guilt connected with the charge on trial; hence it is not always easy at any given moment to decide whether a witness is an accomplice, e.g., for the purpose of corroboration."
Some jurisdictions define an accomplice within their criminal statute. For example, §306 of Chapter 18 of the Pennsylvania Code:
"A person is an accomplice of another person in the commission of an offense if (1) with the intent of promoting or facilitating the commission of the offense, he solicits such other person to commit it; or aids or agrees or attempts to aid such other person in planning or committing it; or (2) his conduct is expressly declared by law to establish his complicity."
In that context, consider this from Justice Baer of the Supreme Court of Pennsylvania in Commonwealth of Pennsylvania v Rega:
"An accomplice is one who actively and purposefully engages in criminal activity and is criminally responsible for the criminal actions of his/her co-conspirators which are committed in furtherance of the criminal endeavor.
"Accordingly, two prongs must be satisfied for a person to be labeled an accomplice. First, there must be evidence that the person intended to aid or promote the underlying offense. Second, there must be evidence that the person actively participated in the crime by soliciting, aiding, or agreeing to aid the principal.
"Further, a person cannot be an accomplice simply based on evidence that he knew about the crime or was present at the crime scene. There must be some additional evidence that the person intended to aid in the commission of the underlying crime, and then aided or attempted to aid."
- Commonwealth of Pennsylvania v. Rega, 933 A. 2d 997 (2007)
- Korell v. State, 253 SW 3d 405 (2008)
- MacDonald v The King,  2 SCR 90
- People v. Williams, 181 P. 3d 1035 (Supreme Court of California, 2008)
- R. v Morris,  2 S.C.R. 1041
- R v Williams,  2 DLR 696 (British Columbia County Court, 1935)
- State v. Chacon, 186 P. 3d 670 (2008)
- Wigmore on Evidence, Volume 3, page 2756