Excerpt:
"Defining the middle class is a perpetually popular parlor game because it's well-known that the foundation of widespread prosperity is a broad-based middle class and a sturdy ladder of social mobility that enables those below the middle class to work their way up to middle class security.

Here's an example of a typical trope on the subject: What Does It Take To Be Middle Class?

The topic is also a perennial favorite because the middle class is losing ground. By basic measures of income, it's slipped from 60% of the populace to 50%.

A strong case can be made that assessed by characteristics of middle class security and prosperity rather than income, the middle class has effectively shrunk to 10% of households as only the top 30% of households earn enough to afford what was within reach of the top 60% in decades past.
...
Regardless of income, households that are living paycheck to paycheck don't qualify as middle class if we qualify "middle class" by these characteristics:

In Why the Middle Class Is Doomed (April 2012) I listed five "threshold" characteristics of membership in the middle class:

1. Meaningful healthcare insurance (i.e. not phantom coverage that only kicks in after thousands of dollars are paid in cash).

2. Significant equity (25%-50%) in a home or other real estate.

3. Income/expenses that enable the household to save at least 6% of its net income.

4. Significant retirement funds: 401Ks, IRAs, etc.

5. The ability to service all debt and expenses over the medium-term if one of the primary household wage-earners lose their job.

I then added a taken-for-granted sixth:

6. Reliable vehicles for each wage-earner that are fully covered by insurance.

Author Chris Sullins suggested adding these additional thresholds:

7. If a household requires government assistance (SNAP, Medicaid, rent subsidies, etc.) to maintain the family lifestyle, their middle class status is in doubt.

8. A percentage of non-financial hard assets such as family heirlooms, precious metals, business equity, rental income property, land, etc. that can be transferred to the next generation, i.e. generational wealth.

9. Ability to invest in offspring (education, extracurricular clubs/training, etc.).

10. Leisure time devoted to the maintenance of physical/spiritual/mental fitness.

Correspondent Mark G. suggested two more:

11. Continual accumulation of human and social capital (adding new skills, expanding social networks and markets for one's services, etc.)

And the money shot:

12. Family ownership of income-producing assets such as rental properties, bonds, etc."