Ambiguity on the factum

What does Ambiguity on the factum mean in American Law?

The definition of Ambiguity on the factum in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

A form of ambiguity in legal writing which goes not just to the meaning of certain language, but to its very legal identity, e.g., whether a particular writing was meant to be a codicil to a will, or whether an entire clause, otherwise almost certainly to be expected, was accidentally left out of a will.