Here's something I learned about 10 minutes ago. According to OED online, conciseness has just one definition: "Concise quality; brevity, terseness." Concision has three definitions:
- The action of cutting to pieces or cutting away; mutilation.
- In Phil. iii. 2 (from the Geneva version of 1557 onward) it translates Gr.
‘cutting off, cutting up’, used there instead of 
‘circumcision’, and applied contemptuously to the Judaizing Christians.[1] - Conciseness.
If you use concision to mean conciseness, the reader will probably figure out the meaning from the context. But use conciseness, and the reader won't have to figure out your meaning; you'll subtract one chance that the reader will misunderstand your message.
_____
1The funny-looking letters are Greek. I hope they show properly on your computer.
was using this tagline on my emails:
concision in style, precision in thought, decision in life -victor hugo-
someone called me on it - said it should be conciseness. but obviously someone translated ol' victor this way....unless he actually said it - don't know if victor hugo spoke l'anglais
Posted by: timm smith | May 05, 2009 at 01:12 PM
True. However, this does not promote conciseness over concision in all situations. There is a wide difference between mutilation, and conciseness.
Posted by: BMR | October 06, 2011 at 07:41 PM