The Feather of Ma'at is the ostrich feather that stands for Ma'at, the ancient Egyptian principle of truth, justice and cosmic order, herself personified as a goddess and, in Jan Assmann's account of the concept, treated in Egyptian thought as both a cosmic force holding the universe in its proper arrangement and a standard of conduct incumbent on every person and above all on the king, whose central duty was understood as maintaining Ma'at against the constant threat of isfet, disorder or wrongdoing.
The feather functions throughout Egyptian art as visual shorthand for the whole of that concept, truth and rightful order together, appearing on its own in inscriptions and iconography wherever a compact reference to Ma'at is wanted without invoking her full personified form. Assmann's broader argument situates Ma'at as unusual among ancient religious concepts for functioning simultaneously as a cosmic principle holding the sun's daily course and the Nile's flood in their proper order and as an entirely ordinary ethical standard invoked in everyday legal and administrative texts, the same word and the same feather doing duty at both scales without any sense in the sources of a contradiction between them.