Definitions for "Rule of thirds"
The rule of thirds is a technique used to place subjects in a photograph. Photographers follow the rule of thirds when they place their subject in a place other than the center of the picture.
This is the basic principle that is the most valuable to a new photographer. This rule takes our rectangular shape and divides it into thirds. The key elements or objects in a composition should fall on one of these thirds lines. The one point on our photograph where the viewers' eye comes to rest should fall on one of these lines where both a horizontal and a vertical line come to rest.
An air management technique. Use a third of the tank to get to your destination, a third to get back, and the last third belongs to your buddy in case of an emergency.
See the Introduction for examples and a diagram.
See the Photography 101 for examples and a diagram.