Pip install python package into a specific directory other than the default install location

I'm using Pip provided by my operating system, which by default installs to '/usr/local/lib/python2.7/dist-packages/'.

I'm using the EPD Python distribution from Enthought, and I would like to install a package for this distribution.

Exactly where do the new files need to be installed; and how can I make Pip install them there?

I found some advice to use:

pip install --install-option="--prefix=$PREFIX_PATH" package_name

but I don't understand what the $PREFIX_PATH environment variable is for (it's currently unset for me).