This issue has never come up in the CICM exam, perhaps because it is the least serious form of traumatic brain injury. However, it comes up in discussion. Specifically, it is important to be able to use this word correctly, knowing precisely what it means, and being able to descend smoothly into academic gibberish (commotio cerebri, etc.) if asked to elaborate.
The majority of the information gathered below comes from the massively authoritative Consensus statement on Concussion in Sport (The 4th International Conference on Concussion in Sport held in Zurich, November 2012). Exploration of this document requires some genuine interest in mild head injury. If one were much less interested, one would be well served by the concise treatment offered to this topic by the LITFL Clinical Case on Minor Head Injury.
Characteristic features defining concussion:
Generally speaking, the terms "concussion" and "mild traumatic brain injury" are used interchangeably.
A more interesting and ICU-worthy question is "why do head-injured patients become unconscious?" Quite apart from parenchymal damage, which should cause focal deficits only, there are some mechanisms at work here which disrupt consciousness on a grand global scale. These mechanisms (or the limitations of our understanding thereof) are discussed in a nice 2010 review article by Blyth and Bazarian. This article, in turn, references an older article by Nigel Shaw, which is also available as full-text.
In short, the affected region must be either both cerebral hemispheres or the brainstem reticular activating system. Four major theories exist, none of which are perfect, but among which the convulsive theory is the most satisfying, as it is supported by the strongest evidence.