At McDonald's restaurants, there are two types of onions: Quarter Onions, which are the big hunks of onions that come on bigger sandwiches, such as the Quarter-Pounder with cheese, and Dehydrated Onions, which are the little diced onions that come on hamburgers and cheeseburgers.
They are called 'dehydrated' because they are ordered in a dehydrated form. Water is added to turn them from hard flakes into the onions that you find on your burger.
When somebody wants their order a special way, the order taker has to specify what the customer wants. The order taker uses a little menu that has words like 'w/out' 'only' 'sub' and then shortened names for condiments such as 'ketchp' 'mustrd' 'pckle' 'onions' (for quarter onions) and 'donion' for dehydrated onions.
Many drive thrus have a menu that displays your order as it is entered into the computer. A cheeseburger without onions will appear as:
1 W/OUT DONION