The difference between "sofa" and "couch" is less firm than it used to be. Really, it mostly depends on where you live and when you were born.
What you call a long, upholstered piece of living room furniture depends on where you live and when you were born.
For example, “couch” and “sofa” are currently the most popular names, but my parents called it a davenport because that name was once common in the upper Midwest, which is where my parents grew up. The name came from the A.H. Davenport Company, a manufacturer of this type of furniture in the mid-1800s and into the early 1900s, including pieces that furnished the White House. From a Google Ngram search in books, it looks as if “davenport” peaked in the United States in the mid-1940s after which its use dropped dramatically and then has been stable at a lower rate since around 1970.
Usage of ‘Couch’ and ‘Sofa’
In 2009, I posted a nonscientific online poll asking what you call a long, upholstered piece of furniture, and nearly 4,500 people replied. "Couch" was the clear winner with 71% of the responses, “sofa” was next with 27% of the responses, and all the others had fractions of a percent:
- Couch 71.1%
- Sofa 26.6%
- Chesterfield 0.8%
- Settee 0.8%
- Davenport 0.6%
- Divan 0.3%
Some people told me that “lounge" (which I neglected to include in the poll) is a dominant term in Australia.