Wearables—smartwatches, wristbands, headsets, and the like—make up one of the fastest-growing segments of the tech industry. Just how fast-growing, though, depends on what you count as a wearable.

For market research firm IDC, wearables include five categories: watches, wristbands, eyewear, clothing, and "other." Together, they're expected to achieve sales of 101.9 million units by the end of this year, a 29 percent growth rate over 2015, according to a report released this week.

But even IDC, with its history of authoritative projections on consumer technology sales, acknowledges that the wearables market is somewhat incoherent and will likely change in the future.

"Unlike the smartphone, which consolidated multiple technologies into one device, the wearables market is a collection of disparate devices," IDC analyst Jitesh Ubrani said in a statement. "Watches and bands are and always will be popular, but the market will clearly benefit from the emergence of additional form factors, like clothing and eyewear, that will deliver new capabilities and experiences."

Makers of some of the most successful wearable digital devices, like Apple, don't even use the term at all. A Google search for "wearable" on Apple's website, for instance, turns up little more than a PDF report from April 2015 that explains the company's procedures for applying chemicals to products that come in contact with their users' skin.

Even the smartwatch segment of the wearables market, driven in no small part by the popularity of the Apple Watch, is made up of disparate products. IDC says smartwatches will make up 41 percent of the total wearables market this year, including devices from traditional watchmakers like Fossil, fitness trackers, and of course gadgets like the Apple Watch and Android Wear-powered watches.

By 2020, IDC predicts that smartwatches will have achieved a compound annual growth rate of 27.8 percent. That's compared to just 4.3 percent for wristbands and a whopping 201.2 percent for eyewear.

In other words, the next four years will likely see a dramatic transformation in what kinds of technology people attach to their bodies. Projector, anyone?