Hollandaise is the emulsified mother sauce made from egg yolks, melted butter, and lemon juice.
Its silky texture comes from an emulsion: egg-yolk lecithin helps suspend butterfat and lemon juice, which normally do not mix. Cooks heat and whisk the ingredients gently, because excessive heat can scramble the yolks and break the sauce. Some recipes use a white-wine or vinegar reduction instead of, or alongside, lemon juice.
Hollandaise is strongly associated with eggs Benedict, but it is also served with asparagus, fish, and other vegetables. Its name means “Dutch sauce” in French, although the sauce is generally treated as part of French cuisine. A common mix-up is Béarnaise, which is a related derivative flavored with a vinegar, shallot, and tarragon reduction rather than plain lemon juice.