Addison Bay is the online retailer with leather yoga pants for the woman on the go | Elizabeth Wellington

When entrepreneur Marguerite Adzick noticed last New Year's Day that more than half the women in Rittenhouse Square's ritzy Parc were wearing yoga pants, she knew she was on to something. That's right, we're talking shiny, stretchy yoga pants expertly paired with cozy UGGs, knee-length boots, dramatic capes, or tunic-sweaters.

"It was my lightbulb moment," said Adzick, who at the time was in the midst of building her online retail apparel company for active women, Addison Bay. She hadn't yet quit her full-time gig as producer of Lilly Pulitizer's e-commerce site, but at this moment  over her, Adzick's self-doubt went, "poof."

"Here we were, at one of the chicest places in town, and 99 percent of the people were wearing yoga pants," Adzick said. "My customer was speaking to me."

On Sept. 4 — two days before her 30th birthday — Adzick launched Addison Bay, featuring quite a collection of eclectic athleisure. At first click-through, the website, with its blue background, gives off a Vineyard Vines feel, but one glance at the leather leggings and racer-striped sports bras and the Anthropologie vibe gets real. In other words, these are the pieces you want to wear while sweating next to that cutie on the treadmill at Orange Theory.

Beyond the cute leggings and matching sports bras, it's the all-important third layer that, Adzick says, elevates the power yoga look to brunch-with-the-girls appropriate, like a heather gray, cropped hoodie, a mauve bomber, or a faux Sherpa jacket.

"Third layers are the game-changers that instantly transform the mood of your outfit," Adzick said. "The modern woman is wearing active wear 24/7, and she wants to be fashion-forward and stylish. So what we are giving her is a one-stop shop."

Whether runway slick Alexander Wang or inspirational Spiritual Gangster, athletic-inspired clothing — or athleisure — continues to take up more space in our closets even if we don't spend our mornings at the gym. That's because comfort is still the driving force behind our wardrobes.

That said, Addison Bay is far from the only online destination for women who want workout gear that's as modern as it is functional. That's why Kate Hudson's Fabletics still dominates our Facebook feeds. Like Addison Bay,  Carbon 38, Bandier, and even Nordstrom, for that matter, offer an array of brands that sell skintight, just-for-the-bike pieces reminiscent of 1990s clubwear.

Despite the growing competition, Adzick is pretty sure Addison Bay will carve out its own corner of the market because she's edgier than websites offering clothes that go from the gym to brunch, despite the steep prices.  Most leggings are about $100, there are sports bras that cost $125, and jackets are upward of $300. She's probably right.

According to the NPD Group, active wear represents 24 percent of the total apparel industry and is forecast to grow through 2019 because not only are we wearing yoga pants and sweatshirts to the office — some companies are making bespoke versions — new yoga and spin studios are popping up in city centers and surrounding gentrified neighborhoods every day. In other words, says NPD chief industry retail adviser Marshal Cohen, athleisure is not going anywhere anytime soon.

Adzick earned her degree in communications from the University of Virginia before landing a job at Lilly in 2012. When Adzick realized she and her friends were building their wardrobes around their workout schedules, she knew she had a business on her hands. She spent two years researching,  writing a business plan, visiting trade shows, and designing not only a website but also a collection of private-label Addison apparel and accessories, including water bottles and cropped sweatshirts.

She raised  more than $100,000 from family and friends and quit her job in April to focus on Addison Bay full time. These days, she spends her days shuttling her 6-month-old daughter from place to place, hosting Addison Bay pop-up events —  where items like the liquid leggings sell out (I'd better hurry up and get my pair), and penning handwritten notes that she includes in every order she takes to the post office.

"I'm on the move, and my clothes are for women on the move," Adzick says. "It's that simple."

(For more information on Addison Bay, log on to addisonbay.com)