common denominator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Mathematics A quantity into which all the denominators of a set of fractions may be divided without a remainder.
• noun A commonly shared theme or trait.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mathematics Any integer that is a common multiple of the denominators of two or more fractions
• noun by extension A trait or attribute that is shared by all members of some category

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an integer that is a common multiple of the denominators of two or more fractions
• noun an attribute that is common to all members of a category