The file .pgpass in a user's home directory or the file referenced by PGPASSFILE can contain passwords to be used if the connection requires a password (and no password has been specified otherwise). On Microsoft Windows the file is named %APPDATA%\postgresql\pgpass.conf (where %APPDATA% refers to the Application Data subdirectory in the user's profile).
This file should contain lines of the following format:
Each of the first four fields may be a literal value, or *, which matches anything. The password field from the first line that matches the current connection parameters will be used. (Therefore, put more-specific entries first when you are using wildcards.) If an entry needs to contain : or \, escape this character with \. A hostname of localhost matches both host (TCP) and local (Unix domain socket) connections coming from the local machine.
The permissions on .pgpass must disallow any access to world or group; achieve this by the command chmod 0600 ~/.pgpass. If the permissions are less strict than this, the file will be ignored. (The file permissions are not currently checked on Microsoft Windows, however.)