die Mayo
mayo
Colloquial short form of mayonnaise, a creamy sauce made from egg yolk, oil, and vinegar or lemon juice.
Ich hätte gern eine Portion Pommes mit Ketchup und Mayo.
I'd like an order of fries with ketchup and mayo.

mayo
Loading your saved state.
mayo
Colloquial short form of mayonnaise, a creamy sauce made from egg yolk, oil, and vinegar or lemon juice.
Ich hätte gern eine Portion Pommes mit Ketchup und Mayo.
I'd like an order of fries with ketchup and mayo.

'Mayo' is the usual and natural form in everyday speech, especially when ordering food. Using the full word 'Mayonnaise' at a snack bar can sound a bit stiff or overly formal.
Einmal Currywurst mit Pommes und extra viel Mayo, bitte!
One currywurst with fries and extra mayo, please!
'Mayo' is a colloquial short form of 'die Mayonnaise' (mayonnaise). Such shortenings are very common in German to make long or foreign-language words easier to pronounce in everyday speech.
Ich mag keine Mayonnaise, also bestelle ich meine Pommes ohne Mayo.
I don't like mayonnaise, so I order my fries without mayo.
The most classic combination in Germany is 'Pommes mit Ketchup und Mayo' (fries with ketchup and mayo). Sometimes this is also called 'Pommes rot-weiß' (literally: red-and-white fries).
Für mich bitte eine Portion Pommes rot-weiß.
For me, an order of fries with ketchup and mayo, please.
mayonnaise
'die Mayo' and 'die Mayonnaise' mean almost the same thing: a thick, cold sauce eaten, for example, with fries, sandwiches, or salad. The difference is mainly in style. 'Mayo' is the short, colloquial form and sounds very natural in everyday conversations. 'Mayonnaise' is the full form and is more likely to appear on packaging, in recipes, or when speaking more precisely and a bit more formally. They are easy to confuse because both refer to the same product. In practice: in everyday life, people often say 'Mayo'; in official or written texts, 'Mayonnaise' is more common.

Zu den Pommes nehme ich heute nur ein bisschen Mayo.
With my fries today, I'll just have a little mayo.
Meine Nachbarin kauft im Supermarkt Mayonnaise für den Kartoffelsalat.
My neighbor is buying mayonnaise at the supermarket for the potato salad.