The unicursal hexagram is a six-pointed star that can be drawn in one continuous movement. Its points are not equidistant, and the lines are not of equal length (unlike a more standard hexagram). It can, however, fit inside a circle with all six points touching the circle.
The unicursal hexagram is commonly depicted with a five-pedaled flower in the center. This is a variation created by Aleister Crowley, and is most strongly associated with the religion of Thelema. Another variation is the placement of a small pentagram in the hexagram's center.
For more information, please see the full article about the hexagram, which includes a diagram about how to construct a unicursal hexagram