I would start with the SEC definition instead of the one currently used:
Asset allocation involves dividing an investment portfolio among different asset categories, such as stocks, bonds, and cash. The process of determining which mix of assets to hold in your portfolio is a very personal one. The asset allocation that works best for you at any given point in your life will depend largely on your time horizon and your ability to tolerate risk.
I would follow with a simplified version of the current statement as suggested by scone:
"Asset allocation is one of the more important decisions investors make. Selection of individual securities is less significant than the overall allocation to stocks, bonds, and cash."
Follow with a footnote reference to the Intelligent Investor:
in the late nineteen eighties, Gary Brinson, a noted money manager and financial analyst and his colleagues published two sophisticated statistical studies of eighty two large pension funds, but concluded that asset allocation accounted for over ninety percent of the return variability among the funds with less than ten percent contribution from market timing and actual stock and bond selection. In other words, asset allocation policy was ten times as important as stock picking and market timing combined" William Bernstein, the Intelligent Asset Allocator.
You could also footnote Sharpe. Nobel Laureate William F. Sharpe writes:
"It is widely agreed that asset allocation accounts for a large part of the variability in the return on a typical investor's portfolio."
This approach defines the topic, makes a statement about its importance, and provides a reference (or two) to support the statement.