A pinch, a dash and a smidgen, have all historically been a small, indefinite amount. In recent years however, several companies have begun selling tiny measuring spoons with the atypical cooking designations smidgen, pinch and dash. Using these as a guide, the precise measurements for those terms follows.
SMIDGEN - A smidgen holds 1/2 pinch or 1/32 teaspoon. 2 smidgens = 1 pinch
PINCH - A pinch holds 1/2 dash or 1/16 teaspoon. 2 pinches = 1 dash
A pinch has historically been defined as "an amount that can be taken between the thumb and forefinger" but without any definite equivalent in other units of measurement.
DASH - A dash holds 1/8 teaspoon. 8 dashes = 1 teaspoon
A dash was originally considered a liquid measure, a small but indefinite amount. More recently the term has been used as both a liquid and dry measurement.