Ribbon Stage is a cooking term used to measure how well egg yolk and sugar are beaten together. Occasionally, a more other-worldly food writer will refer to it as a "sabayon consistency."
The purpose of reaching this stage in beating is both to ensure that enough air has been incorporated into the mixture, and to ensure that the sugar is thoroughly dissolved in the egg yolk so that the egg mixture, when heated, won't become granular.
To reach this stage requires beating the egg yolk for 3 to 4 minutes with a whisk -- less time with an electric beater.
When the Ribbon Stage is reached, there are two clues:
(1) The texture has been changed because a certain quantity of air has been introduced. It will be thick, but lighter than when you started. An implement, such as a whisk or spoon, moved through the mixture leaves a trail behind that is visible for a short while before merging back into the mixture, and if in doing so you expose the bottom of the bowl, you should be able to see the bottom of the bowl for a split-second or so before it fills back in. When you lift the whisk or spoon into the air with some of the mixture on it, the mixture will fall back into the bowl in ribbons, which only slowly disappear back into the mixture.
(2) The yellow colour from the egg yolk will turn pale yellow, its deeper yellow colour having been tempered by the air introduced into it.
How fast the Ribbon Stage is reached depends on what temperature you are working at, and what implement you are using: from slowest to fastest are a spoon, a whisk, a rotary whisk, an electric beater. When using an ordinary whisk, it takes about 3 minutes. When doing it over a double-boiler or water bath (as for some Genoise recipes and for sabayon), the additional heat causes the ribbon stage to arrive earlier. When whipped electrically, froth may appear as well at the same time that the ribbon stage is reached.
It's uncertain how useful a term Ribbon Stage actually is, as very few people outside the ramified world of cooking terminology seem to actually know what it means. Witness the number of recipes which say "until ribbon stage", then immediately follow with an explanation in brackets of what they are actually looking for. They might just as well have dropped the "Ribbon Stage" reference and got on with saying directly what the recipe maker should look for.
The sugar used will be a refined sugar, such as caster sugar, granulated white sugar, or powdered icing sugar.
The eggs may be whole eggs, or egg yolks.
Some recipes will have you beat egg yolks on their own to a ribbon stage, introducing a sugar or sugar syrup afterwards.
Flavourings (such as vanilla, lemon zest or juice, salt, liqueurs) that don't interfere with it reaching a ribbon stage can be added to the mixture.
The term is also occasionally applied to cream that is being whipped.
The most well-known whipped egg yolk dish is perhaps Zabaglione.
The next stage past Ribbon Stage would be referred to as soft peaks.