Billboard is a term commonly used in games to describe a camera-facing plane. This is a polygon with a texture using transparency, that rotates on various axes to always face the viewer.
Billboards are commonly used for particles, far away trees, grass, clouds, UI elements, etc. This is a common device to get more detail in objects without using a lot of polygons. However, they are flat and may look strange when you move around them.
Billboards can be set up to rotate on only one axis or on combinations of axes. For example, a tree might be set to rotate only on its vertical axis, so it always appears grounded. However a puff of smoke usually looks better if it always stays upright and flat to the view.
One cool trick is to use a curved billboard, which helps give more depth to a single-axis billboard, because like a tree billboard, you can look down on it, so the extra depth helps get rid of that flat look.
Also called Sprite, Imposter, Flag, Kite, Stamp.