A square is a form of this equilateral quadrilateral (7)

I believe the answer is:

rhombus

(Other definitions for rhombus that I've seen before include "Equilateral quadrilateral" , "Skewed square" , "Regular parallelogram" , "Special case of parallelogram" , "Geometrical figure" .)

I've seen this clue in The Sunday Times and The Times.

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