A lineup in the general sense is a law enforcement procedure where an eyewitness or alleged eyewitness is shown photographs of suspects in the case, along with "fillers", other people who have similar appearances but who are not involved in the case. If the witness identifies one of the suspects as a participant in the events he/she claims to have witnessed, it is considered evidence towards the guilt of that person. If the witness identifies a "filler" instead of an actual suspect, it indicates that the witness's identification is not reliable evidence.
In the Duke lacrosse case, the alleged victim, Crystal Gail Mangum, went through several procedures which were either acknowledged by the police to be lineups, or were conducted with obvious procedural similarities to lineups. Part of the controversy over the Durham Police Department's handling of the investigation is that they presented her with two lineups conducted according to proper lineup procedure; when Mangum failed to identify her alleged attackers in these lineups, they then put her through a procedure which they claimed was not designed to identify suspects and therefore did not have to adhere to the accepted procedures for conducting lineups, but used her identification of "suspects" during this procedure to secure indictments and represented to the media that Mangum had "identified" her alleged assailants.