An opening of 1♦ used in much the same way as a phoney 1♣. You will meet this most commonly in conjunction with a strong club system and five-card majors.
If opener cannot start with 1♣ because that would show 16+ or 17+ and cannot start with 1♥ or 1♠ because that would show a five-card suit, 1♦ will be a common opening. A ‘Loose Diamond’ is another name for the method.