The C drive on my server is getting critically low on space.  I see multiple virus definition files for my symantec endpoint small business editon 12.0.  My question is, is each definiton file cumulative ( in which case I can delete some older dated files), or does each file represent only new definition updates?

Each folder will be cumulative. You should be able to clear out the older definitions.

Yes, you can clear the old defination files.

Follow this document

How to clear out corrupted definitions for a Symantec Protection Center and Symantec Endpoint Protection Client manually

http://www.symantec.com/business/support/index?page=content&id=TECH98276

Check this

Low Disk Space issues encountered on systems running either the Symantec Endpoint Protection client or the Symantec Endpoint Protection Manager

Disk Space Management procedures for the Symantec Endpoint Protection Manager

https://www-secure.symantec.com/connect/articles/issue-related-low-disk-space

Hi,

In Small Business Edition have limited options, you can not modify settings to reduce number of revisions to keep like Enterprise Edition.

Please check following article:

How to reduce the number of content revisions stored in Symantec Endpoint Protection Manager\Inetpub\content folder in SEP SBE

http://www.symantec.com/docs/TECH98137 

To reduce the database size (sem5.db) can use this tool.

https://www-secure.symantec.com/connect/downloads/shrink-symantec-endpoint-protection-manager-121-embedded-database