Mayonnaise is considered to be an emulsion or semi-liquid. It is a mix of liquids that otherwise would not mix together. It is highly recommended that you keep your mayonnaise in the refrigerator so it can retain the most ideal texture and taste.
Those who keep their mayonnaise out of the refrigerator will risk liquefying it even more and as a result lessening the quality and degrading the texture along with the taste which in turn will make it less than appetizing. Overall this condiment is classified as a semi-liquid as it is also partially a solid and even a gas when it is left out long enough.