Essentially, cccunnings description is a good one, but I thought an additional measurement for the thickness might be useful. To julienne is to cut food into thin strips measuring:

  • 1/8 x 1/8 x 2.5 inches

or

  • 3mm by 3mm by 6.5 mm

In French cooking, the word "Julienne" is also used to describe a clear vegetable soup to which an assortment of thinly cut vegetables cooked in butter are added.