Affordable housing: While just two words, the concept isn't really as simple as it might sound.

Despite the complexity of the issue, we so often hear the term "affordable housing" thrown around by everyone from politicians to activists to the press, as if it were a single, monolithic thing.

But there are different types of affordable housing, said Chris Herbert, managing director of Harvard’s Joint Center for Housing Studies, leftover from years of housing policies.

It's helpful to think of affordable housing essentially in two camps, Herbert said: housing that is subsidized for low-income residents, and housing that’s "provided by the private market without subsidies, but is affordable to households with more modest means."

"Affordability isn’t an intrinsic quality of a housing unit," Herbert said. "It’s relative to what your income is, and it’s relative to what housing costs in the market."

Subsidized housing — what many people think of when they hear the term "affordable housing," Herbert says — is housing for those with lower incomes. It includes public housing developments and what is known as Section 8 housing. Residents receive vouchers or pay a fixed percentage of their income — no matter how modest it is — in rent.

On the other hand, there's non-subsidized housing geared toward being affordable for middle-class households. But in an area as affluent and expensive as the Greater Boston area, who that covers can be a pretty big tent.

"There’s a misconception that affordable housing is a handout for people who aren’t doing their part," said Herbert. "And I think in a region like Boston, what we’re talking about is people who are, but the market as it’s currently functioning is not providing housing that’s within reach."

Read more: The Big Squeeze: Why Does It Cost So Much To Live In Massachusetts?

"The title affordable housing — it carries kind of a stigma with a lot of people, especially in the middle class," said Janiece Diaz, a program coordinator with the NeighborWorks Home Ownership Center of Central Massachusetts in Worcester.

Diaz said there is a veritable cornucopia of non-subsidized, affordable options aimed squarely at the middle class. Whether folks qualify is typically based on their yearly income relative to the Area Median Income (AMI). The AMI varies in each city and town.

According to the U.S. Department of Housing and Urban Development, for a family of four in New Bedford, the median income is $63,000. In Boston, it’s over $100,000. The AMI for Worcester is $85,500, Diaz said.

One kind of non-subsidized, affordable housing is any housing, at market price, that is made more affordable by one or more affordable housing programs. Some might help you gain access to a low-rate mortgage. Others might provide funds that help with a down payment, or cover closing costs.

Affordable housing might also refer to homes or units that are specifically designated as “affordable.” Some of these are for rent, others for purchase. And while access is restricted to those who make no more than some set percentage of the AMI, Harvard’s Chris Herbert said it’s often a pretty hefty percentage.

"The income level they're targeting is not particularly low," said Herbert. "Eighty percent, 100 percent or maybe even up to 120 percent of AMI."

Right now, a full three-quarters of the people who qualify for subsidized affordable housing can’t access it. And the affordable programs and properties are doing little to bridge the gap for the middle class.

It’s not hard to see why nearly everyone agrees a lack of affordable housing in Massachusetts has become a serious issue. And if it’s this difficult to simply get your head around what “affordable housing” even means, just imagine how hard it is to address it.