Port

If you are stood at the back of the vessel facing forward, the left hand side of the vessel is referred to as Port.

Anything on the left hand side of the vessel (looking forward) is referenced as being to port, for example, a locker on the left hand side would be a port locker. 

We can also reference other vessels in relation to us using the term port, i.e. there is a vessel approaching from our port side.