"A salad dressing is best described as an uncooked sauce and, like all sauces, its role is to enhance the flavor of the food." -- 1997 edition, Joy of Cooking.
The American Code of Federal Regulations (CFR) recognizes and defines three types of salad dressings: French dressing, Mayonnaise and "Salad Dressing" (e.g. all others.) 
Small wonder the regulators dodged the topic. Salad Dressings are so hard to define, that even the Joy of Cooking wasn't completely right. As soon as you trip across the topic of Boiled Dressings, their whole "uncooked sauce" definition falls apart.
To be safer, one might say that a Salad Dressing is anything liquidy  that a cook or diner chooses to put on his or her salad.
While it will certainly be liquidy in order to be able to coat the salad greens, the liquid can be of varying thicknesses. It can be pourable; it can be spoonable (as in mayonnaise.) It can be watery, as in a vinaigrette, or creamy, as in a mayonnaise-based or emulsified sauce.
Creamy dressings are generally used with crisper greens; vinaigrettes with more delicate leaves.
Asians often base their Salad Dressings on fish stock or soy sauce. The Romans used salt and oil as their base.
In the Western world, oil is almost always the basis, and with it, almost always use an acid such as vinegar or lemon juice. The standard oils are olive oil or vegetable oils, but depending on the effect desired, nut oils or other specialized oils such as avocado oils can be used, though they are generally in small portions as a flavouring for the main oil being used.
You can buy commercial dressings now that are low-fat or even fat-free. Commercial ones often contain a starch as well as an emulsifier, a thickener, and stabilizers such as Xanthan gum and guar gum.
 American Code of Federal Regulations. Section 169. Food Dressings and Flavourings. 1 April 2006.
 There's nothing to say that years for now a trend for dusting one's salad leaves with completely dry, powdered "Salad Dressing" might not come about.
The general ratio for an oil-based dressing is 1 part acid to 3 parts oil. The acid can be a vinegar, a citrus juice or verjus.
If you are using a strongly flavoured oil, such as a nut oil, or a specially flavoured oil, don't have all three parts of your oil be that oil: use 1 part of that speciality oil to 2 parts regular oil to make up your 3 parts oil.
If your dressing ends up too puckeringly acidic for your taste or your guests, balance it off by stirring a pinch or two of sugar in it.
The E.R Durkee Company of New York started selling its bottled salad dressing as early as 1877. Another early important seller of bottled salad dressing was Curtice Brothers of Rochester, New York.
Salad DressingsAemono; Boiled Dressing; Caesar Salad; Green Goddess Dressing; Mayonnaise; Miracle Whip; Ranch Dressing; Russian Dressing; Salad Cream; Salad Dressings; Thousand Island Dressing
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