If two liquids emulsify, they become one smooth mix instead of sitting in two layers. You can also emulsify them: you make them mix this way, often by beating, and often with an emulsifier. Oil and vinegar on their own pull apart; with mustard they emulsify into a dressing. Mayonnaise is oil emulsified with egg yolk. The word belongs in the kitchen and on food labels more than in everyday chat.