Berkeley socket that allows data to be exchanged between two processes executing on the same Unix or Unix-like host computer.
Unix domain socket file is located in the /tmp directory.
There is no single, standardized "default" path, but the /tmp directory is often used as a convention for temporary files.
For example:
/tmp/.s.PGSQL.5432
This is an attempt to reference a local unix socket with port 5432 for postgresql.