A call in the course of the bidding auction that is insufficient, out of rotation, or otherwise improper.
For example:

By West, a double, redouble or bid of or below 2♦ would be illegal. Any call by one of the other players would be illegal.
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A call in the course of the bidding auction that is insufficient, out of rotation, or otherwise improper.
For example:

By West, a double, redouble or bid of or below 2♦ would be illegal. Any call by one of the other players would be illegal.