The Weighing of the Heart (Judgment)
Your heart on one pan of the scale, the feather of truth on the other — and a devouring monster waiting.
Also: weighing of the heart · judgment of the dead · negative confession · declaration of innocence · spell 125 · Hall of Two Truths · Ammit · psychostasia
Egyptians believed the dead faced a judgment. The heart — seat of thought and conscience — was weighed on a great balance against the feather of ma'at (truth), with Anubis tending the scales, Thoth recording, and Osiris presiding. The deceased recited a declaration of innocence, denying a long list of sins. If the heart balanced true, the person passed into the blessed afterlife; a heart heavy with wrong was devoured by the monster Ammit. This scene is the great theme of Book of the Dead spell 125.
In short
The post-mortem judgment in which the deceased's heart is weighed on a balance against the feather of ma'at before Osiris and a tribunal of gods, accompanied by the 'declaration of innocence' (the so-called negative confession).
Where it’s found
Book of the Dead spell 125 (New Kingdom); antecedents in the Coffin Texts
How the idea developed across Egypt
The fully developed weighing scene and the 'declaration of innocence' belong to the New Kingdom Book of the Dead (esp. spell 125), though ideas of a post-mortem reckoning are older (Coffin Texts; allusions earlier). The vignette becomes a standard image on New Kingdom and later funerary papyri.
'Negative confession' is a modern label; the Egyptian text is a declaration of innocence. Osiris, Anubis, Thoth, Ma'at, and Ammit are deities/beings, not persons.