Leggings seem to be the new jeans — and it’s partly because of a growing trend in America referred to as athleisure, which is pretty much exactly what it sounds like: it’s part athletic wear, part leisure wear.

Though the purpose of each type of clothing seems to be at odds (how can workout gear double as clothing to lounge in?), they have combined to create a rather lucrative niche in the retail world.

While the sales of apparel in the United States only grew 2% overall last year, sales of athletic wear specifically grew an impressive 9%, topping out a $33 billion. That growth, according to market research company NPD, is largely due to the popularity of athleisure wear.

According to NPD’s report, women spent 8% more on athletic wear in 2014 than they did the previous year, and opted for more items like skirts and shirts — which can be easily worn outside of the gym.

“Fashion and function must come together to appeal to today’s always on-the-go consumer,” Marshal Cohen, NPD’s chief industry analyst, said in a release.

Cohen’s right — only 7% of people who wear athletic gear only wear it to work out it. Additionally, 85% of people say they wear their workout gear around the house, 65% say they wear it to run errands, and 42% say they wear it to go shopping.

“This merger of ‘fashion and function’ is not just a passing fashion trend; it is rather a response to the changes in the lifestyle of today’s women,”  says Malek Neman, the CEO of NUX activewear. “As long as society continues to demand more of women, the functional importance of women’s clothing will become more important every day.”

According to Forbes, the trend is showing no signs of slowing. Though the trend proves that Americans’ preferences for their athletic wear are shifting, retailers and designers are sure to keep up.