A "jigger" can either be a measuring device used by bartenders, or a unit of measure (typically 1.5 ounces).
As a measuring device, the typical jigger is a metal, hour-glass shaped receptacle, which can measure two different amounts. The larger end would measure a "jigger" (1.5 ounces) and the smaller end would measure a "pony" (1 ounce).
Ounces versus ml
For cocktail recipes measured in "ounces" it is common to use 1/4 ounce increments, which is how virtually all jiggers are set up with. Here is a comparison of 1/4 ounce increments to their milliliter equivalent.
- Design For A Jigger (C. P. Dungan, 1893) (Google Patents)