Birdie. Par. Bogey. These are all vital terms to scoring in the game of golf. So is eagle. While rare, this score on a hole is quite the achievement, and does happen. The definition and understanding of the term eagle is simple enough to grasp.
Eagle, as defined in golf, is a score of two strokes under par on a hole. As a verb, to eagle is to shoot 2-under par on any given hole.
Eagles don't happen often, but they tend to happen on par-5s. This is because, should a golfer hit two long shots to start the hole, that player then can be faced with a long putt or chip on his third shot. Should the golfer hole that shot, he records an eagle. To do so on a par-4 is quite difficult as it requires holing in on the second shot.
Often, when a hole-in-one is recorded, that shot doubles as an eagle. That is because most holes-in-one take place on par-3 holes. Should a golfer hole in on the tee shot, that golfer would record a 1, recording 2-under par on the hole. Again, this is a rare occurrence, even for professionals, but it does happen.
According to Scottish golf lore, the term eagle was coined as an extension of the term birdie. It was coined as a way of saying big birdie, since an eagle is such a large bird.
Even rarer than an eagle is a double-eagle, in which a golfer shoots 3 under par on a hole. This is only possible on par-4 or longer holes. On par-4s, it requires a golfer to hit a hole-in-one, which almost never happens. On par-5s, it requires a golfer to hole her second shot, which, again, is an extremely rare occurrence. These are not possible on par-3 holes since it's not possible to shoot 3-under par on such a hole.