Winter ice

Winter Ice is both the power of Winter and a physical substance.

As Power
Harry Dresden can call upon this power when he becomes the Winter Knight. He describes it as “Cold, clear power,” which can augment his spells and remove his weariness when called upon, though it is more effective when creating ice, as Harry demonstrated when he used it to transform the floor of Lake Michigan into an ice sheet which lifted him and his friends out of the water. All Winter Faeries have it, and they are susceptible to it, making even the Winter Queen no stronger than the average Faery when facing the Winter Knight.

As Substance
As a substance it is not so different from normal ice, though it dissipates faster, and is more easily summoned by creatures of Winter, as Harry begins to use Ice more frequently.