There is no difference between mayonnaise and salad dressing.

Mayonnaise (salad dressing or mayonnaise) has a history of about 200 years, but there are several theories of its origin: some say that the word mayonnaisa may have evolved from the Old French word moyen (or mayen, meaning egg yolk), referring to the use of egg yolks in the preparation of sauces; others say that it was invented by a French general's chef in honor of his master's victory in the war and named after the General mac mahon's name.

Another theory is that it was invented in the 18th century by the inhabitants of the Spanish island of minorca, but it was named after the method of using olive oil, egg yolks, and lemon juice that was introduced to Spain from France via the island's port of mahon.

Extended information:

Salad dressings are classified according to the ingredients used, as "light" or "heavy" dressings. The "heavy" salad dressings. Light" salad dressings, also known as vinaigrette, are made from white vinegar, olive oil, peanut oil, or soybean oil and other vegetable oils in proportion to the modulation process can be added to salt, pepper and mustard and other seasonings, and can also add fresh herbs and so on, to make the "light" salad dressings.

Heavy salad dressings are made with a base of vegetable oil and white vinegar, as well as fresh egg yolks. Heavy" salad dressings can also be mixed with different spices to make different flavors of "creamy" salad dressings.

Consumers in China with the salad dressing, most of the "creamy" salad dressing, a more accurate name should be mayonnaise in English for Mayonnaise, some people also translated as "mayonnaise''.

Baidu Encyclopedia - Salad Dressing