The key to a great vinaigrette is emulsification. An emulsion is simply a cohesive combination of two liquids that don’t ordinarily mix (like oil and water). In a vinaigrette, those two liquids are oil and, typically, vinegar or lemon juice. The only way to mix them properly is to combine them strenuously enough so that the oil breaks down into such tiny droplets that they remain separated and surrounded by the vinegar droplets.
Thus the two liquids become one. Such oil-in-liquid combos are the most common type of kitchen emulsion, but they’re not the only one. Butter and peanut butter are both examples of water-in-oil emulsions: Their fat content is so high that droplets of water are dispersed throughout the fat.