A CCR is a resource that may prevent the system moving closer towards its goal. A CCR may not be a bottleneck but all CCRs usually need to be taken into account when producing a schedule.
(http://www.toc.co.uk/glossary.htm)
Other Definitions
Capacity Constrained Resource (CCR) – Not necessarily a bottleneck, but a resource that may become a bottleneck if it is not carefully managed .(http://www.tocpractitioner.com/tocglossary.asp)