The phrase "paternal metaphor" is introduced by Lacan in 1957.
In 1958, he goes on to elaborate the structure of this metaphor; it involves the substitution of one signifier (the Name-of-the-Father) for another (the desire of the mother).
The paternal metaphor thus designates the metaphorical (i.e. substitutive) character of the oedipus complex itself.
It is the fundamental metaphor on which all signification depends: for this reason, all signification is phallic.
If the Name-of-the-Father is foreclosed (i.e. in psychosis), there can be no paternal metaphor, and hence no phallic signification.