The definition of "middle class" can vary wildly depending on who you ask. For some, it's defined by certain attributes: If you're hardworking, thrifty and humble, for example, you're middle-class. For others, it means earning a substantial salary but not so much that you'd be considered rich.

That's according to new data from Northwestern Mutual's 2018 Planning & Progress Study, which found that 68 percent of Americans consider themselves middle-class, down 2 percent from last year. However, because of the fuzziness of the definition, far more Americans consider themselves middle-class than technically qualify based on income.

In reality, the middle class now makes up just over 50 percent of the total U.S. population, according to a recent report from Pew Research Center, which used 2016 data. That's compared to 61 percent in 1971.

Between 1971 and 2011, the share of adults in the middle class fell by 10 percent — but since 2011, Pew reports, the middle class has remained relatively stable instead of continuing to shrink.

Pew defines the middle class as those whose annual household income is two-thirds to double the national median, which was $57,617 as of 2016. By that definition, a middle-income three-person household earns about $45,000 to $135,000. If you're single, a salary of around $26,000 to $78,000 qualifies you as middle-income.