What does the title "Phantom Thread" mean?
Is this some tailor craftsmanship jargon? Or was it referenced in the movie itself?
The title refers to the hidden messages that Reynolds stitches into the linings of the dresses he makes.
It's called Phantom Thread because they are messages to a dead woman, written with thread (thread), and only Reynolds knows that it is there (phantom).
His customers wear the dresses completely unaware of it.
He does this because he is haunted by the death of his mother.