A 2-inch wide by 6-inch high (5 by 15 cm) rectangular mark painted on trees and rocks along the trail to mark it.
The entire Appalachian Trail is marked in white-colored blazes.
Intersecting trails are usually marked in blue.
You can see horizontal, diagonal, arrows, and other types of blazes along a trail.
The other kind of blaze doesn't mark the trail but comes after you, and is brightly colored, but not white or blue.