Order imbalances happen on stock exchanges such as the New York Stock Exchange when there are too many buy orders and not enough sell orders -- or vice versa. When they occur, the exchange might halt trading temporarily, to allow more of the other kind of order to come in. This permits better matching of buyers and sellers, and it makes prices less volatile.
You might see order imbalances happen whenever there's very good or bad news related to a company, and suddenly many people want in or out of it.
The Nasdaq operates on more of a supply-and-demand basis, with its trading conducted among many market participants. It doesn't halt trading for order imbalances.
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