This had the following definitions:
http://docs.oracle.com/cd/E19120-01/open.solaris/816-5137/mtintro-75924/index.html

Parallelism 	
A condition that arises when at least two threads are executing  
simultaneously.

Concurrency 	
A condition that exists when at least two threads are making progress. A  
more generalized form of parallelism that can include time-slicing as a  
form of virtual parallelism.

For me it seems like Concurrency is a subset of Parallelism by those  
definitions, as if two threads are executing simultaneously, then it  
follows that two threads are making progress in an overlapping time period.

I know that there are many different definitions, so I am happy with  
whatever we take.