An ounce is a unit of measurement used to measure the weight or volume of something. It is smaller than a pound and is commonly used to measure ingredients in cooking or small quantities of liquid or solids. Full definition
For every pound that you weigh you need to drink a half of ounce of water per pound. (susanciminelli.com)
It may seem expensive for such a tiny bottle, but you have to remember that herbal extracts require many pounds of herb per ounce of fluid extracted... thus increasing the price. (amazon.com)