Definition #1: ADR: The act of recording another reading of the production track in post production. Usually the actor will be looking at the picture on a screen and will be hearing a series of beeps in a headphone giving a countdown to the beginning of the line. (source: Larry Blake) Also see Looping.*also known ADR, Looping and Post Sync.

Definition #2: Dialogue recorded after production to replace unwanted production dialogue. Loop lines are necessary if words are indistinguishable; if the reading of the actor is poor; if the dialogue is overridden by some extraneous sound; or if the background sound is alien to the sequence (e.g., an airplane overhead during a western scene). The original production track and its corresponding picture is made into loops. The actor watches himself and listens to his dialogue over headphones and repeats the dialogue trying to synchronise his lips movements. (source: M.P.S.E)

During looping, it sometimes is necessary to modify the lines of dialogue, usually during scenes where the actor's mouth is not fully visible. Quite a bit of time is spent during dialogue pre-mixing to ensure that the tonal quality of the ADR matches the dialogue recorded on the set. Any change in dialogue character or quality could distract an audience. Good ADR is truly an art form. It requires actors to repeat, not only their lines, but their emotions days or even weeks after the scene is shot. Once looping has been completed, the Dialogue Editor will then conform the “Production Audio” (the live sound) and the ADR to form a unified track. (source: M.P.S.E)

The (Dialogue) Editor takes the newly recorded dialogue and, with the aid of previously indicated sync marks (they) can, more or less easily, replace the old dialogue with the new. (source: M.P.S.E)