A question I thought I knew the answer to...  

Well. . .
it could be said that neither of those specifically addrsses itself to modern design ?
Here's one I recently found, from the first paragraph of Thomas L Doremus, "Classical Styles in Modern Architecture" (1994; Van Nostrand Reinhold):
"The term "modern" has, in the twentieth century, come to denote a highly sophisticated, intellectual interpretation of architectural form based on human perception of three-dimensional space and emphasizing the technology of its construction."
I know there are many others. . .

I would say
A movement in architecture and furniture designed to make peoples lives better through efficency and usability. Focuses were on honest use of materials, functional design over decoration, and utilizing mass production to maximize affordability and distribution.

Here's my best shot...
Modernism is an ideology and so not much worth defining, because ideologies are unchallengeable belief systems--assumed truths if you will. Defining modernism is like defining capitalism, socialism, communism, naziism, neoconservatism, and liberalism. They are all metaphysical concepts that refer to nothing but persons claiming to hold such metaphysical concepts. Circular. Defining an -ism just gives a moniker to an ideology and a moniker neither characterizes usefully, nor explains systematically anything concrete. Who cares what Naziism was? We only care what persons who claimed to be Nazis did. By their deeds shall ye know them.
Defining a modernist design on the other hand refers to a deed--to something concrete, verifiable and capable of being analysed. The modernist ethic was perhaps best expressed in concrete language by an American poet who said: "not ideas about the thing, but the thing itself."
My definition is...
A modernist design is an artifact highly rationalized towards practicality of function, efficiency of materials and simplicity of form that is made from contemporary materials by contemporary methods and refers to nothing but itself.
Many, perhaps most, modernist designs fall short in part or whole to achieve the criteria this definition identifies, but these are still the basic criteria by which a modernist design ought ultimately to be judged.