It's one side 'passing' the move. Denoted by --. Needless to say this is not a legal chess manuever but can be useful in software if you need to animate a sequence of moves made by one side.
What is a "null move"?
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Jan 30, 2013
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#3
A null move is a "non-move" used when reviewing a position using a computer. It is typically used where it doesn't make any difference what the next move is (computers have a nasty habit of requiring that something be entered). Entering a null move allows the review to continue without having to enter a specific move. Many times, a null move is annotated "any."
What is a "null move"?
Answers are appreciated.