Vinaigrette is an oil-based salad dressing or marinade. When a restaurant server asks what kind of dressing you'd like on your salad, vinaigrette is usually one of the options.
A basic vinaigrette is made from some kind of oil (often olive oil) whisked with an acidic ingredient, usually vinegar. The word comes from the French, a variation on vinaigre, or "aromatic vinegar." You can also make a vinaigrette using lemon juice. For some people, a more familiar name is "Italian salad dressing."