A smidgen.
http://www.unc.edu/~rowlett/units/dictS.html
Smidgen
a very small quantity of material. Until recently, no one thought a smidgen was an actual unit of measure, but recently kitchen supply stores in the U.S. and other countries have begun selling sets of “minispoons” in which the smallest spoon, labeled “smidgen,” is designed to hold exactly 1/2 pinch or 1/32 teaspoon, which is roughly 0.005 fluid ounce or 0.15 milliliter. The word is a diminutive of “smutch” or “smudge”; it originally meant a small spot.