An action that isn't against the rules, but still incorporates unfair tactics. Such a maneuver is usually on the border between legality and illegality, though is clearly unethical and performed to take unfair advantage of another player.
Angle is short for Angle Shooting.
The Tournament Director Association (TDA) has formed many new rules in recent year to combat specific angles. Especially the verbal declaration rule that was misused by many angle shooters (players performing an angle) has been made more specific.
An angle isn't illegal, but many tournament directors take a stance when they suspect a player is trying to pull an angle on another player at the table. Often, the first time a player is given a warning. More severe penalties are introduced when the same or other angles occur by the same player.
For some examples of "angles" or "angle shooting," see "Seven Dirty Poker Tricks (and How to Fight Back)."