What is Middle Class?

I can't devote anywhere near a third of my income to discretionary spending, but I consider myself lower middle-class.
Interesting article about econoics in the predominantly Islam countries, and it's probable relation to extremism.

http://www.newsweek.com/id/219341

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In the 1960s, on average no more than a third of the populations of large Muslim countries such as Turkey, Iran, or Pakistan lived in cities, and by most estimates no more than 6 percent of the populations counted as middle class. Today, around two thirds of the populations of those countries live in urban areas, and on average, twice as many count as middle class. If you define the group as those who have a regular income and formal employment with a steady salary and benefits, and who can afford to devote a third of their income to discretionary spending, the middle class now amounts to around 15 percent of the population of Pakistan and twice that in Turkey. The numbers are even higher if you broaden the definition to include those who have adopted modern family values, especially the desire to have fewer children and to invest in their advancement. One estimate puts as many as 60 percent of Iranians in, or ready to enter, that group.

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Something the article doesn't mention is that economic advancement can be expected to lessen extremism and terrorist activity in these countries, we can expect that economic decline would have the opposite effect, including in our own country.