# What exactly is the well defined phase of the BCS ground state wavefunction in superconductivity?

After studying the BCS theory of superconductivity, I do not understand this quote from my notes:

"The phase of the BCS wavefunction is well defined."

I do not see this "phase" from the coherent state form of the BCS ground state. What is it?

It just means that in the BCS ground state, all the electrons form bosonic pair states that condense in a single wavefunction, which hase a phase $\Phi_0$. In this case you can say that all the electrons "share" the same phase $\Phi_0$.