The term is not regulated in most states, so companies can define it as they wish. Peapod, the online shopping service owned by the international grocery giant Ahold, delivers farm-sourced boxes throughout the Northeast; FreshDirect offers a variety of C.S.A. options in and around New York City.

As the “farm share” concept has spread, the C.S.A. has become just another part of the sprawling, messy modern system of knowing where your food comes from and choosing what you want to eat.

The opportunity for confusion is of enormous concern to many farmers in the New York region. Depending on how and where these new businesses buy their produce, consumers can receive all the benefits of C.S.A. membership, while the farmers get only a fraction. Some farmers say that after years of steady growth, their C.S.A. memberships have dropped since the arrival of services like Local Roots or Farmigo.

“At first it seemed like these services were going to be great for us,” said Eve Kaplan-Walbrecht, an owner of Garden of Eve farm on the North Fork of Long Island. Some of them supplied software management programs and marketing tools; others offered premium prices close to farmers’ market rates; others picked up the produce instead of requiring farmers to deliver to a central warehouse.

But the drawbacks can outweigh the benefits. Some say that these hubs have siphoned off their members, partly by offering a more convenient product, but also by blurring the definition of terms like “C.S.A.” and “farm share,” so that customers believe they are directly supporting local farms with their purchases when they might not.

“They are absolutely in competition with us,” said Ben Shute, an owner of Hearty Roots farm in the Hudson Valley. “There are only a certain number of people who will buy food this way.”